Activated microglia do not increase 18 kDa translocator protein ( TSPO ) expression in the multiple sclerosis brain

Abstract Background Accurate and sensitive imaging biomarkers are required to study the progression of white matter (WM) inflammation in neurodegenerative diseases. Radioligands targeting the translocator protein (TSPO) are considered sensitive indicators of neuroinflammation, but it is not clear how well the expression of TSPO coincides with major histocompatibility complex class II (MHCII) molecules in WM. This study aimed to test the ability of TSPO to detect activated WM microglia that are immunohistochemically positive for MHCII in rat models of prodromal Alzheimer’s disease and acute subcortical stroke. Methods 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, a WM region known to have high levels of MHCII activated microglia in TgAPP21 rats. Wild-type rats subsequently received an endothelin-1 (ET1) subcortical stroke and were imaged at days 7 and 28 post-stroke before immunohistochemistry of TSPO, GFAP, iNOS, and the MHCII rat antigen, OX6. Results [18F]FEPPA PET was not significantly affected by genotype in WM and only detected increases near the ET1 infarct (P = 0.033, infarct/cerebellum uptake ratio: baseline = 0.94 ± 0.16; day 7 = 2.10 ± 0.78; day 28 = 1.77 ± 0.35). Immunohistochemistry confirmed that only the infarct (TSPO cells/mm2: day 7 = 555 ± 181; day 28 = 307 ± 153) and WM that is proximal to the infarct had TSPO expression (TSPO 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). Conclusion 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. This suggests that research in neuroinflammation, particularly in the WM, would benefit from MHCII-sensitive radiotracers.

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Translocator Protein 18 kDa (TSPO) Expression in Multiple Sclerosis Patients

Journal of Neuroimmune Pharmacology ◽

10.1007/s11481-012-9397-5 ◽

2012 ◽

Vol 8 (1) ◽

pp. 51-57 ◽

Cited By ~ 17

Author(s):

Erin Harberts ◽

Dibyadeep Datta ◽

Selby Chen ◽

Jillian E. Wohler ◽

Unsong Oh ◽

...

Keyword(s):

Multiple Sclerosis ◽

Translocator Protein ◽

Translocator Protein 18 Kda ◽

Multiple Sclerosis Patients ◽

Tspo Expression

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Expression of Translocator Protein and [18F]-GE180 Ligand Uptake in Multiple Sclerosis Animal Models

Cells ◽

10.3390/cells8020094 ◽

2019 ◽

Vol 8 (2) ◽

pp. 94 ◽

Cited By ~ 11

Author(s):

Anne Nack ◽

Matthias Brendel ◽

Julia Nedelcu ◽

Markus Daerr ◽

Stella Nyamoya ◽

...

Keyword(s):

Multiple Sclerosis ◽

Corpus Callosum ◽

Pet Imaging ◽

Cellular Localization ◽

Grey Matter ◽

Axonal Injury ◽

Cell Types ◽

Translocator Protein ◽

Double Labelling ◽

Tspo Expression

Positron emission tomography (PET) ligands targeting the translocator protein (TSPO) represent promising tools to visualize neuroinflammation in multiple sclerosis (MS). Although it is known that TSPO is expressed in the outer mitochondria membrane, its cellular localization in the central nervous system under physiological and pathological conditions is not entirely clear. The purpose of this study was to assess the feasibility of utilizing PET imaging with the TSPO tracer, [18F]-GE180, to detect histopathological changes during experimental demyelination, and to determine which cell types express TSPO. C57BL/6 mice were fed with cuprizone for up to 5 weeks to induce demyelination. Groups of mice were investigated by [18F]-GE180 PET imaging at week 5. Recruitment of peripheral immune cells was triggered by combining cuprizone intoxication with MOG35–55 immunization (i.e., Cup/EAE). Immunofluorescence double-labelling and transgene mice were used to determine which cell types express TSPO. [18F]-GE180-PET reliably detected the cuprizone-induced pathology in various white and grey matter regions, including the corpus callosum, cortex, hippocampus, thalamus and caudoputamen. Cuprizone-induced demyelination was paralleled by an increase in TSPO expression, glia activation and axonal injury. Most of the microglia and around one-third of the astrocytes expressed TSPO. TSPO expression induction was more severe in the white matter corpus callosum compared to the grey matter cortex. Although mitochondria accumulate at sites of focal axonal injury, these mitochondria do not express TSPO. In Cup/EAE mice, both microglia and recruited monocytes contribute to the TSPO expressing cell populations. These findings support the notion that TSPO is a valuable marker for the in vivo visualization and quantification of neuropathological changes in the MS brain. The pathological substrate of an increase in TSPO-ligand binding might be diverse including microglia activation, peripheral monocyte recruitment, or astrocytosis, but not axonal injury.

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A quantitative neuropathological assessment of translocator protein expression in multiple sclerosis

Brain ◽

10.1093/brain/awz287 ◽

2019 ◽

Vol 142 (11) ◽

pp. 3440-3455 ◽

Cited By ~ 9

Author(s):

Erik Nutma ◽

Jodie A Stephenson ◽

Rianne P Gorter ◽

Joy de Bruin ◽

Deirdre M Boucherie ◽

...

Keyword(s):

Multiple Sclerosis ◽

Protein Expression ◽

Translocator Protein ◽

Inflammatory Phenotype ◽

The Brain ◽

Tspo Expression

Radioligands targeting the 18 kDa translocator protein (TSPO) are increasingly used to visualise inflammation in the brain. Nutma et al. report that TSPO expression in multiple sclerosis lesions originates mainly from astrocytes and microglia, but is not restricted to cells with a specific pro-inflammatory phenotype.

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Heterogeneity of Microglia Phenotypes: Developmental, Functional and Some Therapeutic Considerations

Current Pharmaceutical Design ◽

10.2174/1381612825666190722114248 ◽

2019 ◽

Vol 25 (21) ◽

pp. 2375-2393 ◽

Cited By ~ 5

Author(s):

Yun Yuan ◽

Chunyun Wu ◽

Eng-Ang Ling

Keyword(s):

Brain Lesion ◽

Translocator Protein ◽

Trophic Factors ◽

Microglial Cells ◽

Brain Maturation ◽

Activated Microglia ◽

Proinflammatory Mediators ◽

Pathological Conditions ◽

Positron Emission ◽

M2 Microglia

Background: Microglia play a pivotal role in maintaining homeostasis in complex brain environment. They first exist as amoeboid microglial cells (AMCs) in the developing brain, but with brain maturation, they transform into ramified microglial cells (RMCs). In pathological conditions, microglia are activated and have been classified into M1 and M2 phenotypes. The roles of AMCs, RMCs and M1/M2 microglia phenotypes especially in pathological conditions have been the focus of many recent studies. Methods: Here, we review the early development of the AMCs and RMCs and discuss their specific functions with reference to their anatomic locations, immunochemical coding etc. M1 and M2 microglia phenotypes in different neuropathological conditions are also reviewed. Results: Activated microglia are engaged in phagocytosis, production of proinflammatory mediators, trophic factors and synaptogenesis etc. Prolonged microglia activation, however, can cause damage to neurons and oligodendrocytes. The M1 and M2 phenotypes featured prominently in pathological conditions are discussed in depth. Experimental evidence suggests that microglia phenotype is being modulated by multiple factors including external and internal stimuli, local demands, epigenetic regulation, and herbal compounds. Conclusion: Prevailing views converge that M2 polarization is neuroprotective. Thus, proper therapeutic designs including the use of anti-inflammatory drugs, herbal agents may be beneficial in suppression of microglial activation, especially M1 phenotype, for amelioration of neuroinflammation in different neuropathological conditions. Finally, recent development of radioligands targeting 18 kDa translocator protein (TSPO) in activated microglia may hold great promises clinically for early detection of brain lesion with the positron emission tomography.

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Translocator Protein PET Imaging for Glial Activation in Multiple Sclerosis

Journal of Neuroimmune Pharmacology ◽

10.1007/s11481-010-9243-6 ◽

2010 ◽

Vol 6 (3) ◽

pp. 354-361 ◽

Cited By ~ 75

Author(s):

Unsong Oh ◽

Masahiro Fujita ◽

Vasiliki N. Ikonomidou ◽

Iordanis E. Evangelou ◽

Eiji Matsuura ◽

...

Keyword(s):

Multiple Sclerosis ◽

Pet Imaging ◽

Glial Activation ◽

Translocator Protein

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De novo Neurosteroidogenesis in Human Microglia: Involvement of the 18 kDa Translocator Protein

International Journal of Molecular Sciences ◽

10.3390/ijms22063115 ◽

2021 ◽

Vol 22 (6) ◽

pp. 3115

Author(s):

Lorenzo Germelli ◽

Eleonora Da Pozzo ◽

Chiara Giacomelli ◽

Chiara Tremolanti ◽

Laura Marchetti ◽

...

Keyword(s):

Neurotrophic Factor ◽

De Novo ◽

Neuroactive Steroids ◽

Translocator Protein ◽

Compensatory Mechanism ◽

Human Microglia ◽

De Novo Biosynthesis ◽

Murine Microglia ◽

Synthetic Ligand ◽

Tspo Expression

Neuroactive steroids are potent modulators of microglial functions and are capable of counteracting their excessive reactivity. This action has mainly been ascribed to neuroactive steroids released from other sources, as microglia have been defined unable to produce neurosteroids de novo. Unexpectedly, immortalized murine microglia recently exhibited this de novo biosynthesis; herein, de novo neurosteroidogenesis was characterized in immortalized human microglia. The results demonstrated that C20 and HMC3 microglial cells constitutively express members of the neurosteroidogenesis multiprotein machinery—in particular, the transduceosome members StAR and TSPO, and the enzyme CYP11A1. Moreover, both cell lines produce pregnenolone and transcriptionally express the enzymes involved in neurosteroidogenesis. The high TSPO expression levels observed in microglia prompted us to assess its role in de novo neurosteroidogenesis. TSPO siRNA and TSPO synthetic ligand treatments were used to reduce and prompt TSPO function, respectively. The TSPO expression downregulation compromised the de novo neurosteroidogenesis and led to an increase in StAR expression, probably as a compensatory mechanism. The pharmacological TSPO stimulation the de novo neurosteroidogenesis improved in turn the neurosteroid-mediated release of Brain-Derived Neurotrophic Factor. In conclusion, these results demonstrated that de novo neurosteroidogenesis occurs in human microglia, unravelling a new mechanism potentially useful for future therapeutic purposes.

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Cellular sources of TSPO expression in healthy and diseased brain

European Journal of Nuclear Medicine and Molecular Imaging ◽

10.1007/s00259-020-05166-2 ◽

2021 ◽

Author(s):

Erik Nutma ◽

Kelly Ceyzériat ◽

Sandra Amor ◽

Stergios Tsartsalis ◽

Philippe Millet ◽

...

Keyword(s):

Cell Activity ◽

Brain Regions ◽

Cellular Level ◽

Translocator Protein ◽

Disease Stage ◽

Animal Models Of Disease ◽

Positron Emission ◽

Neuropsychiatric Diseases ◽

Tspo Expression

AbstractThe 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.

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Radiosynthesis,In VivoBiological Evaluation, and Imaging of Brain Lesions with [123I]-CLINME, a New SPECT Tracer for the Translocator Protein

Disease Markers ◽

10.1155/2015/729698 ◽

2015 ◽

Vol 2015 ◽

pp. 1-11 ◽

Cited By ~ 2

Author(s):

F. Mattner ◽

M. Quinlivan ◽

I. Greguric ◽

T. Pham ◽

X. Liu ◽

...

Keyword(s):

Radiochemical Yield ◽

Brain Lesions ◽

Spect Imaging ◽

Translocator Protein ◽

Promising Candidate ◽

The Mean ◽

Iodine 123 ◽

Excitotoxic Lesion ◽

Tspo Expression

The high affinity translocator protein (TSPO) ligand 6-chloro-2-(4′-iodophenyl)-3-(N,N-methylethyl)imidazo[1,2-a]pyridine-3-acetamide (CLINME) was radiolabelled with iodine-123 and assessed for its sensitivity for the TSPO in rodents. Moreover neuroinflammatory changes on a unilateral excitotoxic lesion rat model were detected using SPECT imaging. [123I]-CLINME was prepared in 70–80% radiochemical yield. The uptake of [123I]-CLINME was evaluated in rats by biodistribution, competition, and metabolite studies. The unilateral excitotoxic lesion was performed by injection ofα-amino-3-hydroxy-5-methylisoxazole-4-propionic acid unilaterally into the striatum. The striatum lesion was confirmed and correlated with TSPO expression in astrocytes and activated microglia by immunohistochemistry and autoradiography.In vivostudies with [123I]-CLINME indicated a biodistribution pattern consistent with TPSO distribution and the competition studies with PK11195 and Ro 5-4864 showed that [123I]-CLINME is selective for this site. The metabolite study showed that the extractable radioactivity was unchanged [123I]-CLINME in organs which expresses TSPO. SPECT/CT imaging on the unilateral excitotoxic lesion indicated that the mean ratio uptake in striatum (lesion : nonlesion) was 2.2. Moreover, TSPO changes observed by SPECT imaging were confirmed by immunofluorescence, immunochemistry, and autoradiography. These results indicated that [123I]-CLINME is a promising candidate for the quantification and visualization of TPSO expression in activated astroglia using SPECT.

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