A single-domain antibody for the detection of pathological Tau protein in the early stages of oligomerization.

BACKGROUND: Soluble oligomeric forms of Tau protein have emerged as crucial players in the propagation of Tau pathology in Alzheimer's disease (AD). Our objective is to introduce a single-domain antibody (sdAb) named 2C5 as a novel radiotracer for the efficient detection and longitudinal monitoring of oligomeric Tau species in the human brain. METHODS: The development and production of 2C5 involved llama immunization with the largest human Tau isoform oligomers of different maturation states. Subsequently, 2C5 underwent comprehensive in vitro characterization for affinity and specificity via Enzyme-Linked Immunosorbent Assay and immunohistochemistry on human brain slices. Technetium-99m was employed to radiolabel 2C5, followed by its administration to healthy mice for biodistribution analysis. RESULTS: 2C5 exhibited robust binding affinity towards Tau oligomers (Kd = 6.280 nM ± 0.557) and to Tau fibers (Kd = 5.024 nM ± 0.453), with relatively weaker binding observed for native Tau protein (Kd = 1791 nM ± 8.714) and amyloid peptide (Kd > 10,000 nM). Remarkably, this SdAb facilitated immuno-histological labeling of pathological forms of Tau in neurons and neuritic plaques, yielding a high-contrast outcome in AD patients, closely mirroring the performance of reference antibodies AT8 and T22. Furthermore, 2C5 SdAb was successfully radiolabeled with 99mTc, preserving stability for up to 6 h post-radiolabeling (radiochemical purity > 93%). However, following intravenous injection into healthy mice, the predominant uptake occurred in kidneys, amounting to 115.32 ± 3.67, 97.70 ± 43.14 and 168.20 ± 34.52% of injected dose per gram (% ID/g) at 5, 10 and 45 min respectively. Conversely, brain uptake remained minimal at all measured time points, registering at 0.17 ± 0.03, 0.12 ± 0.07 and 0.02 ± 0.01% ID/g at 5, 10 and 45 min post-injection respectively. CONCLUSION: 2C5 demonstrates excellent affinity and specificity for pathological Tau oligomers, particularly in their early stages of oligomerization. However, the current limitation of insufficient blood-brain barrier penetration necessitates further modifications before considering its application in nuclear medicine imaging for humans.

CCR5 deficiency: Decreased neuronal resilience to oxidative stress and increased risk of vascular dementia.

INTRODUCTION: As the chemokine receptor5 (CCR5) may play a role in ischemia, we studied the links between CCR5 deficiency, the sensitivity of neurons to oxidative stress, and the development of dementia. METHODS: Logistic regression models with CCR5/apolipoprotein E (ApoE) polymorphisms were applied on a sample of 205 cognitively normal individuals and 189 dementia patients from Geneva. The impact of oxidative stress on Ccr5 expression and cell death was assessed in mice neurons. RESULTS: CCR5-Δ32 allele synergized with ApoEε4 as risk factor for dementia and specifically for dementia with a vascular component. We confirmed these results in an independent cohort from Italy (157 cognitively normal and 620 dementia). Carriers of the ApoEε4/CCR5-Δ32 genotype aged ≥80 years have an 11-fold greater risk of vascular-and-mixed dementia. Oxidative stress-induced cell death in Ccr5-/- mice neurons. DISCUSSION: We propose the vulnerability of CCR5-deficient neurons in response to oxidative stress as possible mechanisms contributing to dementia.

Low-dose brain irradiation normalizes TSPO and CLUSTERIN levels and promotes the non-amyloidogenic pathway in pre-symptomatic TgF344-AD rats.

Preclinical studies have recently evaluated the impact of low-dose brain radiation therapy (LD-RT) in animal models of Alzheimer's disease (AD) showing anti-amyloid and anti-inflammatory effects of this treatment. Its effectiveness varied, however, depending on the LD-RT protocol used and the stage when the treatment was applied. In this study, we aimed to evaluate the therapeutic potential of 10 Gy delivered in five daily fractions of 2 Gy (a protocol previously shown to induce an improvement of cognitive performances) in 9-month-old TgF344-AD rats, modeling at a pre-symptomatic stage of the disease. We showed that at an early stage, LD-RT was able to lower levels of the 18-kDa translocator protein (TSPO)-mediated neuroinflammation to normal ranges in addition to the secreted CLUSTERIN, another inflammatory protein also involved in Aß aggregation. In addition, we demonstrated that LD-RT reduces all amyloid forms (~ - 60 to - 80%, P < 0.01; soluble and aggregated forms of Aß40, Aß42, and Aßoligomers). Interestingly, we showed for the first time that sAPPα levels were improved by the treatment, showing a higher activation of the non-amyloidogenic pathway, that could favor neuronal survival. The current evidence confirms the capacity of LD-RT to successfully modulate two pathological hallmarks of AD, namely amyloid and neuroinflammation, when applied before symptoms onset.

Pinealectomy and gonadectomy modulate amplitude, but not photoperiodic modulation of Clock gene expression in the Syrian hamster suprachiasmatic nuclei.

The duration of daytime light phase (photoperiod) controls reproduction in seasonal mammals. Syrian hamsters are sexually active when exposed to long photoperiod, while gonadal atrophy is observed after exposure to short photoperiod. The photorefractory period, or photorefractoriness, is a particular state of spontaneous recrudescence of sexual activity that occurs after a long-term exposure to short photoperiod. Expression of core clock genes in the master circadian clock contained in the suprachiasmatic nuclei depends on photoperiodic conditions. Interestingly, the expression of the Clock gene is also modified in photorefractory Syrian hamsters. Since melatonin and testosterone levels in seasonal species are dependent on photoperiod, photoperiodic variations of Clock mRNA levels in the suprachiasmatic clock could be a consequence of these hormonal changes. To test this hypothesis, we analysed the effects of pinealectomy on Clock mRNA changes due to long to short photoperiod transition and of gonadectomy on Clock mRNA levels in photorefractory period. Our data show that the suprachiasmatic integration of the short photoperiod (assessed by a rhythmic expression profile of Clock) is independent of the presence of melatonin. Furthermore, constitutively low expression of Clock observed during the photorefractory period does not require the presence of either melatonin or testosterone. However, we show that both hormones provide positive feedback on average levels of Clock expression. Thus, our data support the hypothesis that daily variations of Clock levels in the suprachiasmatic nuclei are influenced by photoperiodic changes and the time spent in short photoperiod, independently of seasonal modifications of melatonin or testosterone levels.

Cellular sources of TSPO expression in healthy and diseased brain.

The 18 kDa translocator protein (TSPO) is a highly conserved protein located in the outer mitochondrial membrane. TSPO binding, as measured with positron emission tomography (PET), is considered an in vivo marker of neuroinflammation. Indeed, TSPO expression is altered in neurodegenerative, neuroinflammatory, and neuropsychiatric diseases. In PET studies, the TSPO signal is often viewed as a marker of microglial cell activity. However, there is little evidence in support of a microglia-specific TSPO expression. This review describes the cellular sources and functions of TSPO in animal models of disease and human studies, in health, and in central nervous system diseases. A discussion of methods of analysis and of quantification of TSPO is also presented. Overall, it appears that the alterations of TSPO binding, their cellular underpinnings, and the functional significance of such alterations depend on many factors, notably the pathology or the animal model under study, the disease stage, and the involved brain regions. Thus, further studies are needed to fully determine how changes in TSPO binding occur at the cellular level with the ultimate goal of revealing potential therapeutic pathways.

Design of iodinated radioligands for SPECT imaging of central human 5-HT4R using a ligand lipophilicity efficiency approach.

A series of iodinated ligands for the SPECT imaging of 5-HT4 receptors was designed starting from the previously reported hit MR-26132. We focused on the modulation of the piperidine-containing lateral chain by introducing hydrophilic groups in order to decrease the liphophilicity of the new ligands. All the synthesized compounds were tested for their binding affinities on 5-HT4Rs and based on the Ligand Lipophilicity Efficiency approach, compound 13 was further selected for radioiodination with iodine-125 and imaging experiments. Compound 13 showed its ability to displace the specific signal of the reference compound [125I]SB-207710 but no significant detection of [125I]13 was observed in vivo in SPECT experiments.

TSPO and amyloid deposits in sub-regions of the hippocampus in the 3xTgAD mouse model of Alzheimer's disease.

The involvement of the 18kDa translocator protein (TSPO), a marker of neuroinflammation, in Alzheimer's disease (AD) remains controversial. In the present report, we used [125I]-CLINDE, a SPECT TSPO radiotracer never before used in AD, and we investigated the relationship between TSPO and amyloid plaque density (using [125I]-DRM106) in a triple transgenic mouse model of AD (3xTgAD, APPSWE, PS1M146V and TauP301L). Our results show that TSPO increases appear before those of amyloid deposits. Moreover, the different parts of the hippocampus are differentially affected. Indeed, for both TSPO and amyloid, the subiculum is affected earlier and the ventral hippocampus later than the dorsal hippocampus. In the subiculum and the dorsal hippocampus of 3xTgAD mice, a positive correlation between TSPO and of amyloid deposit levels is observed. This data supports the hypothesis that TSPO could be used as a predictive marker of amyloid pathology. In addition, our immunohistochemical data shows a segregation of TSPO in the hippocampus and immunofluorescence imaging revealed a mainly microglial origin of the TSPO expression. Thus, imaging TSPO with CLINDE may be a good alternative to PET radiotracers.

Dual-radiotracer translational SPECT neuroimaging. Comparison of three methods for the simultaneous brain imaging of D2/3 and 5-HT2A receptors.

PURPOSE: SPECT imaging with two radiotracers at the same time is feasible if two different radioisotopes are employed, given their distinct energy emission spectra. In the case of 123I and 125I, dual SPECT imaging is not straightforward: 123I emits photons at a principal energy emission spectrum of 143.1-179.9 keV. However, it also emits at a secondary energy spectrum (15-45 keV) that overlaps with the one of 125I and the resulting cross-talk of emissions impedes the accurate quantification of 125I. In this paper, we describe three different methods for the correction of this cross-talk and the simultaneous in vivo [123I]IBZM and [125I]R91150 imaging of D2/3 and 5-HT2A receptors in the rat brain. METHODS: Three methods were evaluated for the correction of the effect of cross-talk in a series of simultaneous, [123I]IBZM and [125I]R91150 in vivo and phantom SPECT scans. Method 1 employs a dual-energy window (DEW) approach, in which the cross-talk on 125I is considered a stable fraction of the energy emitted from 123I at the principal emission spectrum. The coefficient describing the relationship between the emission of 123I at the principal and the secondary spectrum was estimated from a series of single-radiotracer [123I]IBZM SPECT studies. In Method 2, spectral factor analysis (FA) is applied to separate the radioactivity from 123I and 125I on the basis of their distinct emission patterns across the energy spectrum. Method 3 uses a modified simplified reference tissue model (SRTMC) to describe the kinetics of [125I]R91150. It includes the coefficient describing the cross-talk on 125I from 123I in the model parameters. The results of the correction of cross-talk on [125I]R91150 binding potential (BPND) with each of the three methods, using cerebellum as the reference region, were validated against the results of a series of single-radiotracer [123I]R91150 SPECT studies. In addition, the DEW approach (Method 1), considered to be the most straightforward to apply of the three, was further applied in a dual-radiotracer SPECT study of the relationship between D2/3 and 5-HT2A receptor binding in the striatum, both at the voxel and at the regional level. RESULTS: Average regional BPND values of [125I]R91150, estimated on the cross-talk corrected dual-radiotracer SPECT studies provided satisfactory correlations with the BPND values for [123I]R91150 from single-radiotracer studies: r = 0.92, p < 0.001 for Method 1, r = 0.92, p < 0.001 for Method 2, r = 0.92, p < 0.001, for Method 3. The coefficient describing the ratio of the 123I-emitted radioactivity at the 125I-emission spectrum to the radioactivity that it emits at its principal emission spectrum was 0.34 in vivo. Dual-radiotracer in vivo SPECT studies corrected with Method 1 demonstrated a positive correlation between D2/3 and 5-HT2A receptor binding in the rat nucleus accumbens at the voxel level. At the VOI-level, a positive correlation was confirmed in the same region (r = 0.78, p < 0.01). CONCLUSION: Dual-radiotracer SPECT imaging using 123I and 125I-labeled radiotracers is feasible if the cross-talk of 123I on the 125I emission spectrum is properly corrected. The most straightforward approach is Method 1, in which a fraction (34%) of the radioactivity emitted from 123I at its principal energy spectrum is subtracted from the measured radioactivity at the spectrum of 125I. With this method, a positive correlation between the binding of [123I]IBZM and [125I]R91150 was demonstrated in the rat nucleus accumbens. This result highlights the interest of dual-radiotracer SPECT imaging to study multiple neurotransmitter systems at the same time and under the same biological conditions.

Different effects of chronic THC on the neuroadaptive response of dopamine D2/3 receptor-mediated signaling in roman high- and roman low-avoidance rats.

The Roman high (RHA)- and low (RLA)-avoidance rat sublines have been identified as an addiction-prone and addiction-resistant phenotype based on their high vs. low locomotor responsiveness to novelty and high vs. low ability to develop neurochemical and behavioral sensitization to psychostimulants, respectively. Most studies though have focused on psychostimulants and little is known about the neuroadaptive response of these two lines to cannabinoids. This study investigated the effects of chronic exposure to Δ9 -tetrahydrocannabinol (THC) on dopamine D2/3 receptor (D2/3 R) availabilities and functional sensitivity in the mesostriatal system of RHA and RLA rats. At baseline, RLA rats exhibited higher densities of mesostriatal D2/3R but lower levels of striatal CB1 R mRNA and displayed a lower locomotor response to acute THC as compared to RHAs. Following chronic THC treatment, striking changes in D2/3 R signaling were observed in RLA but not in RHA rats, namely an increased availability and functional supersensitivity of striatal D2/3 R, as evidenced by a supersensitive psychomotor response to the D2/3 R agonist quinpirole. Moreover, in RLA rats, the lower was the locomotor response to acute THC, the higher was the psychomotor response to quinpirole following chronic THC. These results showing a greater neuroadaptive response of RLA vs. RHA rats to chronic THC thus contrast with previous studies showing a resistance to neuroadaptive response of RLAs to psychostimulants, This suggests that, contrasting with their low proneness to psychostimulant drug-seeking, RLAs may exhibit a heightened proneness to cannabinoid drug-seeking as compared to RHA rats.

A single-scan protocol for absolute D2/3 receptor quantification with [123I]IBZM SPECT.

PURPOSE: Molecular imaging of the D2/3 receptor is widely used in neuropsychiatric research. Non-displaceable binding potential (BPND) is a very popular quantitative index, defined as the product of the receptor concentration (Bavail) and the radiotracer affinity for the receptor (1/appKd). As the appKd is influenced by parameters such as the endogenous neurotransmitter dynamics, it often constitutes a confounding factor in research studies. A simplified method for absolute quantification of both these parameters would be of great interest in this context. Here, we describe the use of a partial saturation protocol that permits to produce an in vivo Scatchard plot and thus estimate Bavail and appKd separately, through a single dynamic SPECT session. To validate this approach, a multi-injection protocol is used for the full kinetic modeling of [123I]IBZM using a 3-tissue compartment, 7-parameter model (3T-7k). Finally, more "classic" BPND estimation methods are also validated against the results of the 3T-7k. METHODS: Twenty-nine male rats were used. Binding parameters were estimated using the 3T-7k in a multi-injection protocol. A partial saturation protocol was applied at the region- and voxel-level and results were compared to those obtained with the 3T-7k model. The partial saturation protocol was applied after an adenovirus-mediated D2 receptor striatal overexpression and in an amphetamine-induced dopamine release paradigm. The Simplified Reference Tissue Model (SRTM), the Logan's non-invasive graphical analysis (LNIGA) and a simple standardized uptake ratio (SUR) method were equally applied. RESULTS: The partial saturation experiments gave similar values as the 3T-7k both at the regional and voxel-level. After adenoviral-mediated D2-receptor overexpression, an increase in Bavail by approximately 18% was observed in the striatum. After amphetamine administration, a 16.93% decrease in Bavail (p<0.05) and a 39.12% increase (p<0.01) in appKd was observed. BPND derived from SRTM, LNIGA and SUR correlated well with the Bavail values from the 3T-7k (r=0.84, r=0.84 and r=0.83, respectively, p<0.0001 for all correlations). CONCLUSION: A partial saturation protocol permits the non-invasive and time-efficient estimation of Bavail and appKd separately. Given the different biological phenomena that underlie these parameters, this method may be applied for the in-depth study of the dopaminergic system in translational molecular imaging studies. It can detect the biological variations in these parameters, dissociating the variations in receptor density (Bavail) from affinity (1/appKd), which reflects the interactions of the receptor with its endogenous ligand.

In Vivo Quantification of 5-HT2A Brain Receptors in Mdr1a KO Rats with 123I-R91150 Single-Photon Emission Computed Tomography.

Our goal was to identify suitable image quantification methods to image 5-hydroxytryptamine2A (5-HT2A) receptors in vivo in Mdr1a knockout (KO) rats (i.e., P-glycoprotein KO) using 123I-R91150 single-photon emission computed tomography (SPECT). The 123I-R91150 binding parameters estimated with different reference tissue models (simplified reference tissue model [SRTM], Logan reference tissue model, and tissue ratio [TR] method) were compared to the estimates obtained with a comprehensive three-tissue/seven-parameter (3T/7k)-based model. The SRTM and Logan reference tissue model estimates of 5-HT2A receptor (5-HT2AR) nondisplaceable binding potential (BPND) correlated well with the absolute receptor density measured with the 3T/7k gold standard (r > .89). Quantification of 5-HT2AR using the Logan reference tissue model required at least 90 minutes of scanning, whereas the SRTM required at least 110 minutes. The TR method estimates were also highly correlated to the 5-HT2AR density (r > .91) and only required a single 20-minute scan between 100 and 120 minutes postinjection. However, a systematic overestimation of the BPND values was observed. The Logan reference tissue method is more convenient than the SRTM for the quantification of 5-HT2AR in Mdr1a KO rats using 123I-R91150 SPECT. The TR method is an interesting and simple alternative, despite its bias, as it still provides a valid index of 5-HT2AR density.

A modified simplified reference tissue model for the quantification of dopamine D2/3 receptors with [18F]fallypride images.

Defluorination of [18F]fallypride and accumulation of 18F in skull and glands leads to the contamination of brain structures with spillover activity due to partial volume effects, leading to considerable errors in binding potential estimations. Here we propose a modification of the simplified reference tissue model (SRTM) to take into account the contribution of skull activity to the radioactivity kinetic pattern in cerebellum and target regions. It consists of the introduction of an additional parameter for each volume of interest (sT) and one for the cerebellum (sR), corresponding to the fraction of skull activity contaminating these structures. Using five rat positron emission tomography experiments, we applied the modified SRTM (SRTMc), which resulted in excellent fits. As a relative means of comparison of results, we applied factor analysis (FA) to decompose dynamic data into images corresponding to brain and skull activity. With the skull factor images, we estimated the "true" sT and sR values, ultimately permitting us to fix the sR value. Parameters obtained with the SRTMc were closely correlated with values obtained from FA-corrected data. In conclusion, we propose an efficient method for reliable quantification of dopamine D2/3 receptors with single-injection [18F]fallypride scans that is potentially applicable to human studies where 18F skull accumulation compromises binding parameter estimation.

Small-animal single-photon emission computed tomographic imaging of the brain serotoninergic systems in wild-type and mdr1a knockout rats.

The pharmacokinetic properties of radiotracers are crucial for successful in vivo single-photon emission computed tomographic (SPECT) imaging. Our goal was to determine if MDR1A-deficient animals could allow better SPECT imaging outcomes than wild-type (WT) animals for a selection of serotoninergic radioligands. Thus, we compared the performances of 123I-p-MPPI, 123I-R91150, 123I-SB207710, and 123I-ADAM radioligands, for imaging of their respective targets (5-hydroxytryptamine [5-HT]1A, 5-HT2A, 5-HT4, and serotonin transporter [SERT]), in WT and Mdr1a knockout (KO) rats. With 123I-SB207710, virtually no SPECT signal was recorded in the brain of WT or KO animals. For 123I-p-MPPI, low nondisplaceable binding potentials (BPND, mean ± SD) were observed in WT (0.49 ± 0.25) and KO (0.89 ± 0.52) animals. For 123I-ADAM, modest imaging contrast was observed in WT (1.27 ± 0.02) and KO (1.31 ± 0.09) animals. For 123I-R91150, the BPND were significantly higher in Mdr1a KO (3.98 ± 0.65) animals compared to WT animals (1.22 ± 0.26). The pharmacokinetics of 123I-SB207710 and 123I-p-MPPI do not make them ideal tracers for preclinical SPECT neuroimaging. 123I-ADAM showed adequate brain uptake regardless of Mdr1a expression and appeared suitable for preclinical SPECT neuroimaging in both animal strains. The use of Mdr1a KO animals significantly improved the brain penetration of 123I-R91150, making this animal strain an interesting option when considering SPECT neuroimaging of 5-HT2A receptors in rat.

Low-Dose Radiation Therapy Impacts Microglial Inflammatory Response without Modulating Amyloid Load in Female TgF344-AD Rats.

Background: Low-dose radiation therapy (LD-RT) has demonstrated in preclinical and clinical studies interesting properties in the perspective of targeting Alzheimer's disease (AD), including anti-amyloid and anti-inflammatory effects. Nevertheless, studies were highly heterogenous with respect to total doses, fractionation protocols, sex, age at the time of treatment and delay post treatment. Recently, we demonstrated that LD-RT reduced amyloid peptides and inflammatory markers in 9-month-old TgF344-AD (TgAD) males. Objective: As multiple studies demonstrated a sex effect in AD, we wanted to validate that LD-RT benefits are also observed in TgAD females analyzed at the same age. Methods: Females were bilaterally treated with 2 Gy×5 daily fractions, 2 Gy×5 weekly fractions, or 10 fractions of 1 Gy delivered twice a week. The effect of each treatment on amyloid load and inflammation was evaluated using immunohistology and biochemistry. Results: A daily treatment did not affect amyloid and reduced only microglial-mediated inflammation markers, the opposite of the results obtained in our previous male study. Moreover, altered fractionations (2 Gy×5 weekly fractions or 10 fractions of 1 Gy delivered twice a week) did not influence the amyloid load or neuroinflammatory response in females. Conclusions: A daily treatment consequently appears to be the most efficient for AD. This study also shows that the anti-amyloid and anti-inflammatory response to LD-RT are, at least partly, two distinct mechanisms. It also emphasizes the necessity to assess the sex impact when evaluating responses in ongoing pilot clinical trials testing LD-RT against AD.

Innately low D2 receptor availability is associated with high novelty-seeking and enhanced behavioural sensitization to amphetamine.

High novelty-seeking has been related to an increased risk for developing addiction, but the neurobiological mechanism underlying this relationship is unclear. We investigated whether differences in dopamine (DA) D2/3-receptor (D2/3R) function underlie phenotypic divergence in novelty-seeking and vulnerability to addiction. Measures of D2/3R availability using the D2R-preferring antagonist [18F]Fallypride, and the D3R-preferring agonist [3H]-(+)-PHNO and of DA-related gene expression and behaviours were used to characterize DA signalling in Roman high- (RHA) and low-avoidance (RLA) rats, which respectively display high and low behavioural responsiveness both to novelty and psychostimulant exposure. When compared to RLA rats, high novelty-responding RHAs had lower levels of D2R, but not D3R, binding and mRNA in substantia nigra/ventral tegmental area (SN/VTA) and showed behavioural evidence of D2-autoreceptor subsensitivity. RHA rats also showed a higher expression of the tyrosine hydroxylase gene in SN/VTA, higher levels of extracellular DA in striatum and augmentation of the DA-releasing effects of amphetamine (Amph), suggesting hyperfunctioning of midbrain DA neurons. RHA rats also exhibited lower availabilities and functional sensitivity of D2R, but not D3R, in striatum, which were inversely correlated with individual scores of novelty-seeking, which, in turn, predicted the magnitude of Amph-induced behavioural sensitization. These results indicate that innately low levels of D2R in SN/VTA and striatum, whether they are a cause or consequence of the concomitantly observed elevated DA tone, result in a specific pattern of DA signalling that may subserve novelty-seeking and vulnerability to drug use. This suggests that D2R deficits in SN/VTA and striatum could both constitute neurochemical markers of an addiction-prone phenotype.

Reactive astrocytes mediate TSPO overexpression in response to sustained CNTF exposure in the rat striatum.

The 18 kDa translocator protein (TSPO) is a classical marker of neuroinflammation targeted for in vivo molecular imaging. Microglial cells were originally thought to be the only source of TSPO overexpression but astrocytes, neurons and endothelial cells can also up-regulate TSPO depending on the pathological context. This study aims to determine the cellular origin of TSPO overexpression in a simplified model of neuroinflammation and to identify the molecular pathways involved. This is essential to better interpret TSPO molecular imaging in preclinical and clinical settings. We used lentiviral vectors (LV) to overexpress the ciliary neurotrophic factor (CNTF) in the right striatum of 2-month-old Sprague Dawley rats. A LV encoding for ß-Galactosidase (LV-LacZ) was used as control. One month later, TSPO expression was measured by single-photon emission computed tomography (SPECT) imaging using [125I]CLINDE. The fluorescence-activated cell sorting to radioligand-treated tissue (FACS-RTT) method was used to quantify TSPO levels in acutely sorted astrocytes, microglia, neurons and endothelial cells. A second cohort was injected with LV-CNTF and a LV encoding suppressor of cytokine signaling 3 (SOCS3), to inhibit the JAK-STAT3 pathway specifically in astrocytes. GFAP and TSPO expressions were quantified by immunofluorescence. We measured a significant increase in TSPO signal in response to CNTF by SPECT imaging. Using FACS-RTT, we observed TSPO overexpression in reactive astrocytes (+ 153 ± 62%) but also in microglia (+ 2088 ± 500%) and neurons (+ 369 ± 117%), accompanied by an increase in TSPO binding sites per cell in those three cell populations. Endothelial cells did not contribute to TSPO signal increase. Importantly, LV-SOCS3 reduced CNTF-induced astrocyte reactivity and decreased global TSPO immunoreactivity (-71% ± 30%), suggesting that TSPO overexpression is primarily mediated by reactive astrocytes. Overall, this study reveals that CNTF induces TSPO in multiple cell types in the rat striatum, through the JAK2-STAT3 pathway in astrocytes, identifying this cell type as the primary mediator of CNTF effects neuroinflammatory processes. Our results highlight the difficulty to interpret TSPO imaging in term of cellular origin without addition cellular analysis by FACS-RTT or quantitative immunostainings. Consequently, TSPO should only be used as a global marker of neuroinflammation.

18 kDa Translocator Protein TSPO Is a Mediator of Astrocyte Reactivity.

An increase in astrocyte reactivity has been described in Alzheimer's disease and seems to be related to the presence of a pro-inflammatory environment. Reactive astrocytes show an increase in the density of the 18 kDa translocator protein (TSPO), but TSPO involvement in astrocyte functions remains poorly understood. The goal of this study was to better characterize the mechanisms leading to the increase in TSPO under inflammatory conditions and the associated consequences. For this purpose, the C6 astrocytic cell line was used in the presence of lipopolysaccharide (LPS) or TSPO overexpression mediated by the transfection of a plasmid encoding TSPO. The results show that nonlethal doses of LPS induced TSPO expression at mRNA and protein levels through a STAT3-dependent mechanism and increased the number of mitochondria per cell. LPS stimulated reactive oxygen species (ROS) production and decreased glucose consumption (quantified by the [18F]FDG uptake), and these effects were diminished by FEPPA, a TSPO antagonist. The transfection-mediated overexpression of TSPO induced ROS production, and this effect was blocked by FEPPA. In addition, a synergistic effect of overexpression of TSPO and LPS on ROS production was observed. These data show that the increase of TSPO in astrocytic cells is involved in the regulation of glucose metabolism and in the pro-inflammatory response. These data suggest that the overexpression of TSPO by astrocytes in Alzheimer's disease would have rather deleterious effects by promoting the pro-inflammatory response.

Low-molecular weight sulfated marine polysaccharides: Promising molecules to prevent neurodegeneration in mucopolysaccharidosis IIIA?

Mucopolysaccharidosis IIIA is a hereditary disease caused by mutations in the sulfamidase enzyme that participates in catabolism of heparan sulfate (HS), leading to HS fragment accumulation and multisystemic failure. No cure exists and death occurs around the second decade of life. Two low molecular weight highly sulfated compounds derived from marine diabolican and infernan exopolysaccharides (A5_3 and A5_4, respectively) with heparanase inhibiting properties were tested in a MPSIIIA cell line model, resulting in limited degradation of intracellular HS. Next, we observed the effects of intraperitoneal injections of the diabolican derivative A5_3 from 4 to 12 weeks of age on MPSIIIA mice. Brain metabolism and microstructure, levels of proteins and genes involved in MPSIIIA brain pathophysiology were also investigated. 1H-Magnetic Resonance Spectroscopy (MRS) indicated deficits in energetic metabolism, tissue integrity and neurotransmission at both 4 and 12 weeks in MPSIIIA mice, with partial protective effects of A5_3. Ex-vivo Diffusion Tensor Imaging (DTI) showed white matter microstructural damage in MPSIIIA, with noticeable protective effects of A5_3. Protein and gene expression assessments displayed both pro-inflammatory and pro-apoptotic profiles in MPSIIIA mice, with benefits of A5_3 counteracting neuroinflammation. Overall, derivative A5_3 was well tolerated and was shown to be efficient in preventing brain metabolism failure and inflammation, resulting in preserved brain microstructure in the context of MPSIIIA.

Translocator protein is a marker of activated microglia in rodent models but not human neurodegenerative diseases.

Microglial activation plays central roles in neuroinflammatory and neurodegenerative diseases. Positron emission tomography (PET) targeting 18 kDa Translocator Protein (TSPO) is widely used for localising inflammation in vivo, but its quantitative interpretation remains uncertain. We show that TSPO expression increases in activated microglia in mouse brain disease models but does not change in a non-human primate disease model or in common neurodegenerative and neuroinflammatory human diseases. We describe genetic divergence in the TSPO gene promoter, consistent with the hypothesis that the increase in TSPO expression in activated myeloid cells depends on the transcription factor AP1 and is unique to a subset of rodent species within the Muroidea superfamily. Finally, we identify LCP2 and TFEC as potential markers of microglial activation in humans. These data emphasise that TSPO expression in human myeloid cells is related to different phenomena than in mice, and that TSPO-PET signals in humans reflect the density of inflammatory cells rather than activation state.

Quantification of dopamine D(2/3) receptors in rat brain using factor analysis corrected [18F]Fallypride images.

The goal of this work is to quantify the binding parameters of [(18)F]Fallypride in the striatal and extrastriatal regions of the rat brain using factor analysis (FA) to correct small animal PET kinetic imaging for spillover defluorination radioactivity. Eleven rats were employed for YAP-(S)PET acquisitions and metabolite studies. All kinetic parameters including B'(max) and K(d)V(R) were estimated with a three-tissue compartment seven-parameter model (3T-7k) on the basis of all the FA-corrected data from the multi-injection protocol. Binding potential (BP(ND)) was calculated with Logan's graphical analysis taking cerebellum as the reference region and using the first injection raw (BP(ND-RAW)) and FA-corrected (BP(ND-FA)) data. Three distinct factors corresponding to free+non-specific binding, specific binding and skull and gland accumulation were recovered from FA with their corresponding spatial distributions. The resulting reconstructed images without skull and gland accumulation were improved to provide a better contrast between specific and non-specific regions. Very bad fits were obtained when using time-activity curves (TACs) calculated from the raw [(18)F]Fallypride data, whereas all TACs were well fitted by the 3T-7k model after FA correction. FA-corrected data enables the cerebellar region to be used as reference for the Logan approach. The magnitude of the BP(ND-FA) values was increased from 21% to 108% across regions and the rank order of BP(ND-FA) values (Cx