The translocator protein (Da) and its role in disorders

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Neuroscience and Biobehavioral Reviews

journal homepage:

www.elsevier.com/locate/neubiorev

Review article

The translocator protein (18 kDa) and its role in neuropsychiatric disorders

Tatiana Barichello

a,b,⁎,1

, Lutiana R. Simões

b,1

, Allan Collodel

b

, Vijayasree V. Giridharan

a

,

Felipe Dal-Pizzol

c

, Danielle Macedo

d

, Joao Quevedo

a,e,f,g

a_{Translational Psychiatry Program, Department of Psychiatry and Behavioral Sciences, McGovern Medical School, The University of Texas Health Science Center at}

Houston (UTHealth), Houston, TX, USA

b_{Laboratory of Experimental Microbiology, Graduate Program in Health Sciences, Health Sciences Unit, University of Southern Santa Catarina (UNESC), Criciúma, SC,}

Brazil

c_{Laboratory of Experimental Pathophysiology, Graduate Program in Health Sciences, Health Sciences Unit, University of Southern Santa Catarina (UNESC), Criciúma, SC,}

Brazil

d_{Neuropharmacology Laboratory, Drug Research and Development Center, Department of Physiology and Pharmacology, Faculty of Medicine, Federal University of Ceará,}

Fortaleza (UFC), CE, Brazil

e_{Laboratory of Neurosciences, Graduate Program in Health Sciences, Health Sciences Unit, University of Southern Santa Catarina (UNESC), Criciúma, SC, Brazil} f_{Center of Excellence on Mood Disorders, Department of Psychiatry and Behavioral Sciences, McGovern Medical School, The University of Texas Health Science Center at}

Houston (UTHealth), Houston, TX, USA

g_{Neuroscience Graduate Program, The University of Texas MD Anderson Cancer Center UTHealth Graduate School of Biomedical Sciences, Houston, TX, USA}

A R T I C L E I N F O

Keywords:

TSPO TSPO ligands Neuroinﬂammation Neuropsychiatric disorders

A B S T R A C T

Translocator protein (TSPO) is an 18 kDa translocator membrane protein expressed in the outer mitochondrial membrane of steroid-synthesizing cells in the central and peripheral nervous systems. TSPO is involved in cellular functions, including the regulation of cell proliferation, transport of cholesterol to the inner mi-tochondrial membranes of glial cells, regulation of mimi-tochondrial quality control, and haem synthesis. In the

brain, TSPO has been extensively used as a biomarker of injury and inﬂammation. Indeed, TSPO was

up-regulated in several inﬂammatory and neurodegenerative diseases. In contrast, the expression of TSPO was

decreased in peripheral blood from psychiatric patients. Since TSPO is involved in several mechanisms related to

mitochondrial function and inﬂammatory alterations, therapeutic approaches focusing on the regulation of

TSPO may provide a new avenue for the treatment of neuropsychiatric disorders. Based on the involvement of mitochondrial alterations in the neurobiology of neuropsychiatric disorders, this review will focus on the functions and physiological roles of TSPO and the potential of TSPO ligands as therapeutic strategies for the treatment of neuropsychiatric disorders.

http://dx.doi.org/10.1016/j.neubiorev.2017.10.010

Received 31 May 2017; Received in revised form 20 September 2017; Accepted 10 October 2017

⁎_{Corresponding author at: Department of Psychiatry and Behavioral Sciences, The University of Texas Medical School at Houston, 1941 East Road, Suite 3144, Houston, TX, 77054,}

USA.

1_The

ﬁrst two authors contributed equally to the manuscript.

E-mail address:Tatiana.Barichello@uth.tmc.edu(T. Barichello).

Abbreviations:3α-HSD, 3α-hydroxysteroid dehydrogenase; 3β-HSD, 3β-hydroxysteroid dehydrogenase/Δ5−Δ4; AIF, apoptosis inducing factor; AD, Alzheimer’s disease; ANT, adenine nucleotide translocator; AP-1, activator protein 1; APAF-1, apoptotic peptidase activating factor-1; ASC, caspase-recruitment domain; ATAD-3, AAA domain-containing protein 3; ATP, adenosine triphosphate; BD, bipolar disorder; CCL2, chemokine (C-C motif) ligand 2; CNS, central nervous system; CUS, chronic unpredictable stress; DAA-1097, N-(4-chloro-2-phe-noxyphenyl)-N-(2-isopropoxybenzyl) acetamide; DAA-1106, N-(2,5-dimethoxybenzyl)-N-(5-ﬂuoro-2-phenoxyphenyl) acetamide; DAMPs, damage-associated molecular patterns; DNA,

deoxyribonucleic acid; ERK-1/2, extracellular signal-regulated kinase-1/2; ETS, E-twenty six oncogene homologue; FGIN-1, 2 aryl-3-indoleacetamides; GABAA, gamma-amino-butyric

acid type A receptor; GSP, generalized social phobia; HIV, human immunodeﬁciency virus; IAP, inhibitor of apoptosis protein; IL-1β, interleukin-1β; iNOS, inducible nitric oxide

(NO)-synthase; LPS, lipopolysaccharide; MAPK, mitogen-activated protein kinases; MEK-1/2, mitogen activated protein kinase/Erk kinase1/2; MPIGA, N,N-di-n-propyl-2-(4-methylphenyl) indol-3-ylglyoxylamide; MPTP, mitochondrial permeability transition pore; mRNA, messenger ribonucleic acid; MS, multiple sclerosis; NIX, nip3-like protein X; NLR, NOD-like receptors;

NLRP-3, NLR family pyrin domain containing-3; NMDAR, N-methyl-D-aspartate receptor; NO, nitric oxide; OCD, obsessive-compulsive disorder; P450scc, mitochondrial cytochrome P450

side-chain cleavage enzyme; PAMPs, pathogen-associated molecular patterns; PANDAS, paediatric autoimmune neuropsychiatric disorders associated with streptococcal; PD, Parkinson’s

disease; PARKIN, PTEN-induced putative kinase- NIX; PET, positron emission tomography; PINK-1, PTEN-induced putative kinase-1; PK 11195,

1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinoline carboxamide; PKCε_{, activate the protein kinase-C}ε_{; PTEN, phosphatase and tension homologue deleted on chromosome 10; PTSD, post-traumatic stress}

disorder; Raf-1, direct regulator of MKK1 and MKK2; Ro5-4864, 4′-chlorodiazepam; ROS, reactive oxidative species; SMAC, second mitochondria-derived activator of caspase; SQSTM-1,

sequestosome 1; SSR-180575, 7-chloro-N,N,5-trimethyl-4-oxo-3-phenyl-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide; StAR, steroidogenic acute regulatory protein; STAT-3,

signal transducer and activator of transcription-3; SULT, cytosolic sulfotransferase enzymes; THDOC, tetrahydrodeoxycorticosterone; TNF-α, tumour necrosis factor-α; TSPO, translocator

protein; VDAC, voltage dependent anion channel; XBD-173/AC-5216, N-benzyl-N-ethyl-2-(7,8-dihydro-7-methyl-8-oxo-2-phenyl-9H-purin-9-yl)acetamide; YL-IPA08, N-ethyl-N-(2-pyr-idinylmethyl)-2-(3,4-ichlorophenyl)-7-methylimidazo[1,2-a] pyridine-3-acetamide hydrochloride; ZBD-2, N-benzyl-N-ethyl-2-(7,8-oxo-2-phenyl-9H-purin-9-yl) acetamide

Neuroscience and Biobehavioral Reviews 83 (2017) 183–199

Available online 18 October 2017

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1. Introduction

Translocator protein (18 kDa) (TSPO), is expressed in the outer

mitochondrial membrane of cells in both the central and peripheral

nervous system (Papadopoulos et al., 2006). TSPO has been implicated

in various functions, including apoptosis, regulation of cholesterol

transport, steroidogenesis, cell proliferation, control of many

mi-tochondrial functions, and modulation of the immune system. TSPO

interacts with the mitochondrial permeability transition pore (MPTP),

which is thought to regulate the mitochondrial membrane potential.

The interactions between TSPO and other proteins in the mitochondrial

membrane are related to apoptosis, steroidogenesis, phosphorylation,

the generation of reactive oxygen species (ROS), the production of ATP,

and the collapse of the mitochondrial membrane potential, thus leading

to cell death (Papadopoulos et al., 2017; Veenman et al., 2007).

In

ﬂ

ammation is associated with a marked up-regulation of TSPO in

the mitochondria and drastic changes in the morphology of microglia.

Furthermore, TSPO was found to be up-regulated in Alzheimer’s disease

(AD) (Kreisl et al., 2016), Parkinson

’

s disease (PD) (Gerhard et al.,

2006), multiple sclerosis (MS) (Janssen et al., 2016), traumatic brain

injury (Papadopoulos and Lecanu, 2009), ischaemic stroke (Gerhard

et al., 2005), frontotemporal dementia (Cagnin et al., 2004), human

immunode

ﬁ

ciency virus (HIV) (Zhou et al., 2014), and paediatric

au-toimmune neuropsychiatric disorders associated with streptococcal

in-fection (PANDAS) (Kumar et al., 2015). In contrast, the expression of

TSPO was reduced in platelets from depressed patients (Chelli et al.,

2008; Sarubin et al., 2016), in peripheral mononuclear cells from

an-xious patients (Nudmamud et al., 2000), and bipolar disorder (BD)

patients su

ﬀ

ering from adult separation anxiety (Abelli et al., 2010).

Recently, due to its involvement in neuroin

ﬂ

ammatory alterations, a

positron emission tomography (PET) imaging study explored the potential

role of TSPO as a molecular target in neuroin

ﬂ

ammation and

neurode-generative diseases (Ching et al., 2012; Endres et al., 2009). Thus, based

on the important role of TSPO in neuroin

ﬂ

ammatory and

neurodegen-erative diseases, this review aims to i) enumerate the functions and

phy-siological roles of TSPO, ii) provide evidence that TSPO can serve as an

imaging biomarker of brain neuroin

ﬂ

ammatory disorders, and iii) draw

attention to the ligands of TSPO as a new avenue for mitochondrial-based

therapeutic approaches for the treatment of neuropsychiatric disorders.

2. Physiological properties and apoptotic function of TSPO

TSPO, which was previously denominated as peripheral-type

benzodiazepine receptor (PBR), was renamed to represent more

accu-racy regarding to structure and subcellular functions. These new

no-menclature and abbreviation were de

ﬁ

ned at an international

con-sortium meeting and approved by HUGO Gene Nomenclature

Committee (HGNC) and International Union of Pure and Applied

Chemistry (IUPAC) organizations (Papadopoulos et al., TIPS 2006).

TSPO is a protein that is mainly localized in the outer membrane of

the mitochondria and it is expressed in the brain and extremely

abun-dant in steroid synthesizing tissues and cells such as gonads and adrenal

cells (Braestrup et al., 1977; Mohler and Okada, 1977; Rupprecht et al.,

2010). TSPO is also expressed in heart, kidney, liver, hemopoietic, and

lymphatic cells (Anholt et al., 1985; Bond et al., 1985; Davies and

Huston, 1981; Moingeon et al., 1983; Taniguchi et al., 1982). In the

central nervous system (CNS), TSPO is expressed in microglia, activated

astrocytes, and neurons in the mammalian olfactory bulb (Anholt et al.,

1984; De Souza et al., 1985; Park et al., 1996). TSPO has the capacity to

form a functional and stable polymer that consists of a helical bundle

structure composed of

ﬁ

ve transmembrane helices (Korkhov et al.,

2010). TSPO interacts with voltage-dependent anion channels (VDACs)

to contribute to mitochondrial quality control and the regulation of

both the mitochondrial structure and function (McEnery et al., 1992;

Snyder et al., 1987). Thus, TSPO forms a multimeric complex with

VDACs and adenine nucleotide translocators (ANTs), which are

loca-lized in the outer and inner mitochondrial membranes, respectively.

This mitochondrial multiprotein complex, namely MPTP, is responsible

for the permeability of water and small substances at the junction of the

inner and outer mitochondrial membranes (Kinnally et al., 1993;

McEnery et al., 1993; McEnery et al., 1992). The interaction between

the proteins VDAC and TSPO appears to play a role in apoptotic cell

death (Veenman et al., 2007; Veenman et al., 2008). Interestingly,

MPTP activation increases the permeability of the mitochondrial

membrane, which facilitates the induction of cardiolipin oxidation by

reactive oxidative species (ROS) (Kovacic et al., 1991; Veenman et al.,

2008). Cardiolipin is a phospholipid that is predominantly located in

the inner mitochondrial membrane and is related to the release of

cy-tochrome-c

from the intermembrane spaces into the cytosol.

Con-versely, cytochrome-c can also catalyse cardiolipin, which facilitates

the permeabilization of the outer mitochondrial membrane; thus,

cy-tochrome-c

plays a pro-apoptotic role (Orrenius and Zhivotovsky, 2005;

Petrosillo et al., 2003; Veenman et al., 2008).

Cytochrome-c

is an electron carrier that is localized between

mi-tochondrial respiratory complexes III and IV, and it is anchored to the

mitochondrial

membrane

through

interactions

with

acidic

Fig. 1.MPTP functions in the apoptosis pathway. Cardiolipin can be

oxidized by ROS, which triggers the release of cytochrome-c,allowing

cytochrome-cto enter the cytosol and induce apoptosis. Cytochrome-c

can also catalyse cardiolipin, which facilitates the release of AIF and SMAC through the MPTP into the cytosol. SMAC promotes the acti-vation of caspase-9 by binding to the inhibitor of IAP and removing its inhibitory activity, thus triggering caspase activation in the

cyto-chrome-c/APAF-1/caspase-9 pathway. Abbreviations: AIF, apoptosis

inducing factor; ANT, adenine nucleotide translocator; APAF-1, apoptotic peptidase activating factor-1; BAX, pro-apoptotic protein associated BCL-2; CL-OX, catalysed oxidation; IAP, inhibitor of apoptosis protein; MPTP, mitochondrial permeability transition pore; ROS, reactive oxygen species; SMAC, second mitochondria-derived

activator of caspase; TSPO, translocator protein–18 kDa; VDAC,

vol-tage-dependent anion channel.

T. Barichello et al. Neuroscience and Biobehavioral Reviews 83 (2017) 183–199

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phospholipids, particularly cardiolipin (Orrenius and Zhivotovsky,

2005). Cytochrome-c

binds to apoptotic peptidase activating factor-1

(APAF-1). Thus, this complex recruits and activates pro-caspase-9,

which triggers the activation of caspases 3, 6, and 7 (Li et al., 1997;

Veenman et al., 2008). Cytochrome-c

also facilitates the release of

apoptosis inducing factor (AIF) and second mitochondria-derived

acti-vator of caspase (SMAC) through the MPTP into the cytosol (Twiddy

et al., 2004). Furthermore, SMAC, which is a mitochondrial protein that

can promote the activation of caspase-9 by binding to the inhibitor of

apoptosis protein (IAP) and removing its inhibitory activity, is a

com-ponent of another apoptotic pathway. Thus, SMAC triggers the

activa-tion of caspase through the cytochrome-c/APAF-1/caspase-9 pathway

(Du et al., 2000).

Fig. 1

shows the function of MPTP in the apoptosis

pathway.

Previous studies have revealed that TSPO transfections resulted in

an enrichment of clusters related to translation, transcription, protein

folding, nucleotide excision repair, the electron transport chain,

pro-teasome and up-regulated genes associated with these functions (Liu

et al., 2017). In a rodent model of AD, the

β-amyloid fragment Aβ

(25–35)

was injected into the rat CA1 hippocampal area. The Aβ

(25–35)

fragment

damaged the mitochondrial structure in hippocampal neurons and

in-creased the expression of genes associated with the MPTP, including

VDAC, ANT, and cyclophilin-D (Cyp-D). This study demonstrated that

mitochondrial dysfunction resulted in an increase in MPTP gene

ex-pression, triggering neurodegenerative disorders (Ren et al., 2011). In

addition, in a

Drosophila melanogaster

septic injury model, the

TSPO-VDAC complex, together with the PARKIN protein, mediated the

re-sponses against the infection. Furthermore, the knockdown of TSPO

alone resulted in defective responses to the infection (Cho et al., 2015).

In contrast, evidence has shown that VDAC and ANT do not play a role

in the multiprotein MPTP since a genetic analysis of the MPTP excluded

the genes encoding VDAC and ANT (Baines et al., 2007; Kokoszka et al.,

2004; Selvaraj and Stocco, 2015). In fact, heart and liver mitochondria

from VDAC1-null, VDAC3-null, and VDAC1/VDAC3-null mice exhibited

a Ca

2+

_{and oxidative stress-induced MPTP that was indistinguishable}

from wild type mitochondria (Baines et al., 2007). Furthermore,

mi-tochondria from the mouse liver with a genetic inactivation of two

isoforms of ANT could still undergo a permeability transition, resulting

in the release of cytochrome-c. This study revealed that due to the

ge-netic inactivation of the ANT isoforms, the activation of MPTP required

higher levels of Ca

2+

. These authors concluded that while ANT

con-tributed to the regulation of the MPTP, ANT was a non-essential

structural component of the MPTP (Kokoszka et al., 2004). In contrast,

TSPO is a benzodiazepine receptor that di

ﬀ

ers from the CNS

gamma-aminobutyric acid type A (GABA

A

) receptor, and pharmacological

evidence supports the connection between TSPO and MPTP (Rupprecht

et al., 2010; Selvaraj and Stocco, 2015).

3. TSPO and neurosteroidogenesis

TSPO, which is primarily located in the outer mitochondrial

mem-brane, is highly expressed in steroid-synthesizing tissues, including glial

cells. There are few reports questioned the role of TSPO in

ster-oidogenesis (Banati et al., 2014; Morohaku et al., 2014; Tu et al., 2014),

however, most studies, including the recent ones, argue that TSPO is an

important protein for steroid synthesis (Fan et al., 2015; Frye, 2009;

Papadopoulos and Lecanu, 2009). TSPO is a part of a large multiprotein

complex that also includes VDAC, ATPase, AAA domain-containing

protein-3 (ATAD-3), and steroidogenic acute regulatory protein (StAR).

This complex is involved in the transport of cholesterol to the inner

mitochondrial membrane of glial cells (Midzak et al., 2011; Rone et al.,

2009). The transport of cholesterol is an initial step in the synthesis of

important neuroactive steroids. Then, cholesterol is converted into

pregnenolone by mitochondrial cytochrome P450 side-chain cleavage

enzyme (P450scc) (Costa et al., 1994; Lacapere and Papadopoulos,

2003; Peltola et al., 1996). The endogenous steroid pregnenolone, in

turn, di

ﬀ

uses into the cytosol and is metabolized into di

ﬀ

erent

neu-roactive steroids (Liu et al., 2006). Microsomal 3β-hydroxysteroid

de-hydrogenase/Δ5-Δ4 (3β-HSD) isomerase is responsible for the

meta-bolization of pregnenolone into progesterone. Then, progesterone is

converted into 5α-dihydroprogesterone by 5α-reductase.

5α-dihy-droprogesterone is then converted by 3α-hydroxysteroid

dehy-drogenase (3α-HSD) into allopregnanolone. Pregnenolone synthesis

occurs in cortical and hippocampus pyramidal neurons and

pyramidal-like neurons in the basolateral amygdala (Agis-Balboa et al., 2006;

Melcangi et al., 1994). The steroid tetrahydrodeoxycorticosterone

(THDOC) is synthesized from the adrenal hormone deoxycorticosterone

by the enzymes 5α-reductase and 3α-HSD. Allopregnanolone and

THDOC are positive modulators of GABA

A

receptor-mediated

neuro-transmission, which is related to inhibitory neurobehavioural e

ﬀ

ects.

Pregnenolone sulphate (pregnenolone-S) is formed by the sulphonation

of pregnenolone by the cytosolic sulfotransferase enzymes (SULT)-2A1,

SULT-2B1a and SULT-2B1b (Harteneck, 2013). This neuroactive steroid

is a potent negative allosteric modulator of the GABA

A

receptor and a

positive allosteric modulator of the excitatory N-methyl-D-aspartate

receptor (NMDAR) (Whittaker et al., 2008). In an experimental study,

pregnenolone-S reversed the schizophrenia-like behaviours in

dopa-mine transporter knockout mice (Wong et al., 2015). Oestradiol, which

is derived from testosterone following the aromatization of the A-ring

by the P450 enzyme aromatase, is another import steroid. Oestradiol is

one of the most potent physiological regulators of NMDAR-dependent

plasticity and memory in the hippocampus (Potier et al., 2016).

Fig. 2

explains the role of TSPO in neurosteroidogenesis.

4. TSPO, reactive oxygen species and mitophagy

TSPO-mediated cell protection is directly related to the levels of

oxidative balance. Hence, low levels of ROS induce the expression of

TSPO, which facilitates cholesterol transport and simultaneously

fers protective characteristics to the mitochondrial membrane. In

con-trast, high levels of ROS decrease cholesterol transport and dysregulate

the homeostasis of the mitochondrial membrane by releasing

cyto-chrome-c

(Batarseh and Papadopoulos, 2010; Gatli

ﬀ

and Campanella,

2012). This pathway involves ROS activation of protein kinase-C

ε

(PKC

ε

) through the mitogen-activated protein kinase

(MAPK)/extra-cellular signal-regulated kinase-1/2 (ERK-1/2) pathway, which is also

known as the direct regulator of MKK1 and MKK2 (Raf-1) mitogen

activated protein kinase/Erk kinase1/2 (MEK1/2)

−

ERK1/2 pathway.

The MAPK/ERK pathway signal transduction promotes the gene

ex-pression of TSPO via c-Jun, activator protein 1 (AP-1), E-twenty six

oncogene homologue (ETS), and signal transducer and activator of

transcription-3 (STAT-3) (Batarseh et al., 2010) as shown in

Fig. 3.

TSPO is strongly associated with in

ﬂ

ammation, and its overexpression

is inhibited by Tumour necrosis factor-α

(TNF-α)-induced

mitochon-drial ROS in endothelial cells (Joo et al., 2012). Astrocytes highly

ex-press TSPO mRNA in rat cell cultures under hypo-osmolarity conditions

(Kruczek et al., 2009). Thus, ROS-producing compounds, including

TNF-α, interleukin-1β

(IL-1β) and lipopolysaccharide (LPS), are also

related to the up-regulated expression of TSPO (Bourdiol et al., 1991;

Rey et al., 2000).

Under normal physiological conditions, mitophagy is the selective

segregation and degradation of the mitochondria, which maintains the

function and genetic integrity of these organelles (Gatli

ﬀ

and

Campanella, 2015). Di

ﬀ

erent pathways are involved in the degradation

of mitochondria in mammalian cells; however, the PTEN-induced

pu-tative kinase-1 (PINK-1) and E3 ubiquitin ligase PARKIN (PARKIN)

pathway is well characterized. PINK-1 is a mitochondrial serine/

threonine-protein kinase that is encoded by the PINK-1 gene, which

accumulates at the outer mitochondrial membrane after membrane

depolarization. During this process, PINK-1 is translocated to the

mi-tochondria by the mimi-tochondrial receptor Nip3-like (NIX) protein, and

both proteins recruit PARKIN, which causes the ubiquitination of

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VDAC-1 and results in the removal of depolarized mitochondria via

mitophagy (Deas et al., 2011; Saita et al., 2017). An elegant study

proposed a model of the regulation of cell mitophagy by the expression

ratio of TSPO and VDAC-1. This study revealed that a low TSPO

expression relative to the expression of VDAC-1 causes a decrease in

ROS production and facilitates PARK-2-mediated mitochondrial

ubi-quitination and the recruitment of the autophagosomal machinery.

However, if the TSPO expression is higher than the expression of

VDAC-Fig. 2.TSPO-multiprotein complex increases neurosteroidogenesis. The TSPO complex transports cholesterol to the inner mitochondrial membranes and is converted into pregnenolone by P450scc.

Abbreviations: 3α-HSD, 3α-hydroxysteroid dehydrogenase; 3β-HSD,

3β-hydroxysteroid dehydrogenase/Δ5-Δ4; Ca2+_, _calcium; _Cl−_,

chloride ion; GABAA, gamma-amino-butyric acid type A receptor;

MPTP, mitochondrial permeability transition pore; NMDA,

N-methyl-D-aspartate; P450scc, mitochondrial cytochrome P450 side-chain

cleavage enzyme; SULT, cytosolic sulfotransferase enzymes; THDOC, tetrahydrodeoxycorticosterone.

Fig. 3.Eﬀects of ROS on MPTP. Mitochondrial ROS induced the

ac-tivation of protein kinase-Cε (PKCε) through the MAPK/ERK-1/2

pathway, which is also known as the Raf-1-MEK1/2-ERK1/2 pathway. Thus, the MAPK/ERK pathway signal transduction promotes the gene expression of TSPO via c-Jun, AP-1, ETS, and STAT-3. Abbreviations: AP-1, activator protein-1; c-Jun, c-Jun N-terminal kinases; ERK-1/2, extracellular signal-regulated kinase-1/2; ETS, E-twenty-six oncogene homologue; MEK-1/2, mitogen activated protein kinase/ERK kinase1/

2; MPTP, mitochondrial permeability transition pore; PKCε_{, protein}

kinase-Cε; Raf-1, direct regulator of MKK1 and MKK2; ROS, reactive

oxygen species; START-3, signal transducer and activator of

tran-scription-3; TSPO, translocator protein–18 kDa.

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1, ROS production increases, resulting in the inhibition of the

PARK-2-mediated ubiquitination, sequestosome (SQSTM-1) recruitment and

mitophagy (Gatli

ﬀ

et al., 2014).

5. E

ﬀ

ects of TSPO ligands in pre-clinical models of

neuropsychiatric disorders

Recent studies have elucidated the bene

ﬁ

cial e

ﬀ

ects of TSPO ligands

in pre-clinical models of neuropsychiatric disorders. These ligands

in-clude 1-[2-chlorophenyl]-N-[1-methyl-propyl]-3-iso-quinoline

carbox-amide (PK 11195), 4′-chlorodiazepam (Ro5-4864),

2-ethylamino-6-chloro-4-methyl-4-phenyl-4H-3,1-benzoxazine hydrochloride (etifoxine,

etafenoxine, Stresam

®

_),

7-chloro-N,N,5-trimethyl-4-oxo-3-phenyl-3,5-di-hydro-4H-pyridazino[4,5-b]indole-1-acetamide (SSR-180575),

N-benzyl-N-ethyl-2-(7,8-oxo-2-phenyl-9H-purin-9-yl)acetamide (ZBD-2),

[N-ethyl-N-(2-pyridinylmethyl)-2-(3,4-ichlorophenyl)-7-methylimidazo

[1,2-a]

pyridine-3-acetamide hydrochloride] (YL-IPA08), 2 aryl-3-indoleaceta

mides (FGIN-1-27 and FGIN-1-43),

N-benzyl-N-ethyl-2-(7,8-dihydro-7-methyl-8-oxo-2-phenyl-9H-purin-9-yl) acetamide (known as emapunil,

AC-5216, or XBD-173),

N,N-di-n-propyl-2-(4-methylphenyl)indol-3-yl-glyoxylamide (MPIGA), N-(4-chloro-2-phenoxyphenyl)-N-(2-isopropoxy

benzyl)acetamide (DAA-1097),

N-[(2,5-dimethoxyphenyl)methyl]-N-(5-ﬂ

uoro-2-phenoxyphenyl)acetamide (DAA-1106), and midazolam (Costa

et al., 2011; do Rego et al., 2015; Li et al., 2017a; Li et al., 2014;

Nothdurfter et al., 2012; Okuyama et al., 1999; Ravikumar et al., 2016;

Veiga et al., 2005; Wang et al., 2017), as listed in

Table 1.

The anti-in

ﬂ

ammatory mechanisms of the TSPO ligands could be

explained based on their abilities to reduce the activation of microglia

activation and inhibit transcription factors that are related to the

synthesis of pro-in

ﬂ

ammatory mediators. Thus, PK 11195 decreased the

content of

β-amyloid

(1–42)

, increased the brain levels of progesterone

and allopregnanolone, and protected against cognitive de

ﬁ

cits induced

by chronic LPS administration (Ma et al., 2016). In another study, PK

11195, Ro5-4864, and SSR-180575 decreased the in

ﬂ

ammatory

pul-monary response and alveolitis onset (Bribes et al., 2003). The

combi-nation of Ro5-4864 and PK 11195 reduced the levels of soluble

β-amyloid in the brains of mice submitted to a model of AD (Barron et al.,

2013). The TSPO ligands also had an e

ﬀ

ect on NOD-like receptors

(NLR)-family pyrin domain containing-3 (NLRP-3) in

ﬂ

ammasomes,

which can be activated by damage-associated molecular patterns

(DAMPs), pathogen-associated molecular patterns (PAMPs), ROS, and

cardiolipin (Iyer et al., 2013). In this study, NLRP-3 activates caspase-1,

which proteolytically activates IL-1β

and IL-18 (Latz et al., 2013). The

Ro5-4864 treatment was related to the following actions: i) inhibition

of NLRP-3 in

ﬂ

ammasome activation, ii) suppression of

apoptosis-asso-ciated speck-like protein containing a caspase-recruitment domain

(ASC) oligomerization, iii) decreased release of IL-1β

and IL-18, iv)

reduction in the production of mitochondrial superoxide and v)

main-tenance of the mitochondrial membrane potential triggered by

adeno-sine triphosphate (ATP) in LPS-primed cells culture (Lee et al., 2016).

In an

in vitro

study, the TSPO ligand XBD-173 inhibited the

tran-scription of pro-in

ﬂ

ammatory genes, namely, chemokine (C-C motif)

ligand 2 (CCL2), IL-6 and inducible nitric oxide (NO)-synthase (iNOS).

In addition, the XBD-173 treatment reduced the migratory capacity and

proliferation of microglia (Karlstetter et al., 2014). Etifoxine, which is

an anxiolytic and anticonvulsant drug, modulated the activity of TSPO

by decreasing peripheral immune cell in

ﬁ

ltration in the spinal cord and

increasing oligodendroglial regeneration after in

ﬂ

ammatory

demyeli-nation

in

a

mouse

experimental

model

of

autoimmune

en-cephalomyelitis (Daugherty et al., 2013).

TSPO ligands are also considered important regulators of

ster-oidogenesis and potential therapeutic targets for neurological disorders

(Baez et al., 2017). The results obtained from a study investigating a

TSPO conditional knockout mouse model suggested that TSPO played a

critical role in the preimplantation of the embryogenesis and

hormone-mediated adrenal steroid production (Fan et al., 2015). The exposure of

Table 1 TSPO ligands a ffi nity according to the function tested. Ligands Function tested TSPO a ffi nity E ff ect Reference DAA-1097 Anxiolytic-like e ff ects. Agonist Anxiolytic properties. Okuyama et al. (1999) DAA-1106 Anxiolytic-like e ff ects. Agonist Anxiolytic properties Okuyama et al. (1999) Emapunil (AC-5216 or XBD-173) Anxiolytic-like e ff ects. Agonist Anxiolytic properties. Kita and Furukawa (2008) , Qiu et al. (2013) , Zhang et al. (2017) Increase neurosteroid biosynthesis. Agonist Neurosteroid biosynthesis Kita and Furukawa (2008) , Ravikumar et al. (2016) Decrease in fl ammation. Agonist Anti-in fl ammatory e ff ects Karlstetter et al. (2014) Etifoxine Neurosteroid biosynthesis. Agonist Neurosteroid biosynthesis. do Rego et al. (2015) FGIN-1 – 27 anti-depressive activity. Agonist Anxiolytic drug; neurosteroid biosynthesis. Guillon et al. (2001) FGIN-1 – 43 Anti-depressive activity. Agonist Anxiolytic drug; neurosteroid biosynthesis. Guillon et al. (2001) Midazolam Anxiolytic-like and antidepressant-like e ff ects. Agonist Antidepressant-like and anxiolytic-like e ff ects. Miao et al. (2014) MPIGA Anxiolytic activity. Agonist Neurosteroid production and anxiolytic properties. Costa et al. (2011) PK 11195 Anxiety-like behaviour. Antagonist Exacerbated anxiety-like behaviour. Zhang et al. (2017) Neurosteroid biosynthesis. Antagonist Decreased neurosteroid biosynthesis. do Rego et al. (2015) , Frye and Paris (2011) Ro5-4864 Decrease reactive gliosis and prevent hilar neuronal loss. Agonist Neuroprotective at lower doses. Veiga et al. (2005) ) SSR-180575 Neuroprotective e ff ect. Agonist Promoted neuronal survival and repair. Ferzaz et al. (2002) YL-IPA08 Anxiolytic-like e ff ects. Agonist Antidepressant-like and anxiolytic-like e ff ects. Zhang et al. (2014) ZBD-2 Antidepressant-like and anxiolytic-like e ff ects. Agonist Antidepressant-like and anxiolytic-like e ff ects. Wang et al. (2017)

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frog hypothalamic explants to graded concentrations of etifoxine

pro-duced a dose-dependent increase in the biosynthesis of

17-hydro-xypregnenolone, dehydroepiandrosterone, progesterone and

tetra-hydroprogesterone

and

a

decrease

in

the

production

of

dihydroprogesterone (do Rego et al., 2015). Furthermore, the TSPO

MPIGA ligand had a positive relationship with the induction of

ster-oidogenesis and the oxidative metabolism activity/proliferation index

in human astrocyte cell models (Da Pozzo et al., 2016). In contrast,

some studies suggest that TSPO plays a crucial role in viability and that

steroidogenesis is a misconception related to an older model (Selvaraj

et al., 2016). The mechanism associated with the anxiolytic properties

of the TSPO ligands involves the positive modulation of inhibitory

G-ABAergic neurotransmission (Costa et al., 2011). In PTSD-like mice

treated with YL-IPA08, the levels of allopregnanolone increased in the

prefrontal cortex, which was partially related to the anti-PTSD e

ﬀ

ect (Li

et al., 2014). In an

in vitro

study, an incubation of primary cultures of

rat brain astrocytes with YL-IPA08 increased the concentrations of

pregnenolone and progesterone in the culture medium, while an

in vivo

treatment with YL-IPA08 resulted in antidepressant-like and

anxiolytic-like e

ﬀ

ects in rodent behaviour models (Zhang et al., 2014). ZBD-2 has

a therapeutic e

ﬀ

ect in chronic stress-related disorders, such as

depres-sion and anxiety, by regulating the levels of the biogenic amines and

synaptic proteins in the mouse hippocampus (Wang et al., 2017). In a

rat model of depression induced by chronic unpredictable stress (CUS),

YL-IPA08 suppressed the CUS-induced decrease in locomotor activity,

reduction in sucrose preference and increase in the latency to eat. In

addition, YL-IPA08 treatment increased the levels of progesterone and

allopregnanolone in the hippocampus and prefrontal cortex of the CUS

rats (Li et al., 2017a), as shown in

Table 2.

6. Positron emission tomography (PET) imaging in

neuropsychiatric pre-clinical models

In a PET imaging study evaluating the brains of APP/PS1 transgenic

mice, which is an experimental model of AD, a progressive increase in

[

3

_{H](R)-PK11195 binding and [}

11

_{C](R)-PK11195 retention was}

ob-served

in vivo

using the

μPET technique. This progressive increase in

[

3

_{H](R)-PK11195 binding and [}

11

_{C](R)-PK11195 retention correlated}

with the histopathological abundance of activated microglia (Venneti

et al., 2009). In another study, the [

11

C](R)-PK11195 distribution

vo-lume values in AD mice were signi

ﬁ

cantly higher than those in control

mice after a wash-out period of 15 months. These results were

con-ﬁ

rmed by the immunohistochemistry data. However, [

11

C](R)-PK11195

μPET could not demonstrate genotype- or

treatment-depen-dent di

ﬀ

erences in 13- to 14-month-old animals (corresponding to

ap-proximately 60 years in human age), suggesting that at this age and

disease stage, the microglial activation in the AD mice is too mild to be

detected by this imaging method (Rapic et al., 2013). In an ischaemia

model, [

11

_{C](R)-PK11195 binding was detected at 4 and 7 days in the}

ipsilateral hemisphere of the rat brain (Rojas et al., 2007). In another

study using

in vivo

PET imaging, a minocycline treatment signi

ﬁ

cantly

decreased the uptake [

18

_{F]DPA-714 (a new generation TSPO}

radio-tracer for detecting and monitoring neuroin

ﬂ

ammation) 7 days after

cerebral ischaemia in rats (Martin et al., 2011), as shown in

Table 3.

7. Changes in TSPO expression in neuropsychiatric disorders

As previously mentioned, TSPO and its ligands stimulate

neuro-steroidogenesis by facilitating cholesterol translocation from the outer

mitochondrial membrane to the inner mitochondrial membrane

(Milenkovic et al., 2015). Neurosteroids can potentiate or inhibit

neu-rotransmitter receptor functions; however, neurosteroids generally act

as e

ﬃ

cient positive modulators of GABA

A

receptor function (Rupprecht

et al., 2010). The GABAR molecular structure consists of

ﬁ

ve

homo-logous subunits from a family of 16 genes, including

α

1–6

,

β

1–3

,

γ

1–3

,

δ,

ε

,

π, and

θ, with a distinct organization depending on the brain

location. For example, the

α

1,4,5,6

β

2,3δ

/

ε

subunits are present in

hip-pocampal dentate gyrus cells, cingulate gyrus granular cells, and

tha-lamic ventrobasal nucleus, whereas the

α

5

βγ

2

/δ

subunit is present in

the CA1 sub

ﬁ

eld in the hippocampal area. Thus, depending on the

re-spective receptor subunit composition, neurosteroids may increase or

decrease the in

ﬂ

ux of a chloride current through GABA

A

(Belelli et al.,

2009; Rupprecht et al., 2010), thus modulating the distinct functions of

the receptor.

Although it has been several decades after the establishment of

TSPO upregulation during neuroin

ﬂ

ammation, the role of TSPO and its

signalling mechanisms in regulation of neuroin

ﬂ

ammation is yet to be

elucidated. At this point, it is also imperative to understand the biphasic

role TSPO in neuroin

ﬂ

ammation and neuropsychiatric illness. A study

by Bae et al., investigated the role of TSPO in regulating LPS-induced

microglia activation and neuroin

ﬂ

ammation both

in vivo

and

in vitro.

The results suggest that knock-down of TSPO leads to increased

gen-eration of pro-in

ﬂ

ammatory cytokines such as TNF and IL-6 expression.

On the other hand, the opposite occurred with TSPO over-expression,

the generation of pro-in

ﬂ

ammatory cytokines was attenuated when

compared to control BV2 cells. The authors explain that the increased

expression of TSPO during neuroin

ﬂ

ammation might be an adaptive

response mechanism. Thus, this study provides the molecular level

experimental data demonstrating that TSPO is a negative regulator of

microglia activation and neuroin

ﬂ

ammation (Bae et al., 2014; Najjar

et al., 2013). In addition to neuroin

ﬂ

ammation also in neuropsychiatric

disorders the biphasic role of TSPO has been reported. A decrease in the

mRNA levels of TSPO was observed in lymphocytes from patients with

anxiety, generalized anxiety disorder, PTSD and obsessive-compulsive

disorder (OCD) compared to those in healthy controls (Dell'Osso et al.,

2010; Nudmamud et al., 2000; Rocca et al., 2000; Rocca et al., 1998). In

contrast, the TSPO mRNA levels were normalized after anxiety

treat-ments (Rocca et al., 1993). In a study involving patients with

general-ized social phobia (GSP), the TSPO density in the platelets was lower

than that in the control group (Johnson et al., 1998). This lower level of

platelet TSPO density was evident in a subgroup of bipolar patients,

who also ful

ﬁ

lled the criteria for adult separation anxiety disorder

(Abelli et al., 2010). Furthermore, a decreased platelet TSPO density

was observed in post-Persian Gulf War patients with PTSD (Gavish

et al., 1996) and depressed patients with adult separation anxiety

(Chelli et al., 2008). Finally, schizophrenic patients with aggressive

behaviour presented a lower density of platelet TSPO and scored

sig-ni

ﬁ

cantly higher on hostility, anxiety, state anger, and emotional

dis-tress compared to homicidal and nonaggressive schizophrenic patients

and the controls (Ritsner et al., 2003). A study by Weizman et al.,

de-monstrated that there were no association between depression and

re-duced TSPO expression (Weizman et al., 1995). However, co-morbid

adult separation anxiety (Abelli et al., 2010) or suicidality (Soreni et al.,

1999) was associated with reduced TSPO expression in patients with

depression or bipolar disorder.

Hence deep understanding of biology and pathophysiology of this

promising molecular target, TSPO and its ligands is desired in the

central and peripheral nervous system, to use these TSPO ligands for the

diagnosis and treatment of neurological and psychiatric disorders.

8. TSPO ligands as PET imaging probes

TSPO was unaltered in the brains of individuals with

mild-to-mod-erate depression using PET. [

11

_{C]PBR28, which is a ligand with an}

adequate sensitivity to localize and quantify the associated increase in

TSPOs, was used as a marker of neuroin

ﬂ

ammation (Hannestad et al.,

2013). In another study, a signi

ﬁ

cant reduction in [

11

_{C](R)-PBR28 in}

the grey matter of drug-naive

ﬁ

rst-episode psychotic patients was

de-tected compared with healthy controls (Collste et al., 2017).

Ad-ditionally, the second-generation TSPO PET radioligand [

18

_F]FEPPA,

which was used to evaluate microglial activation in the dorsolateral

prefrontal cortex and hippocampus of untreated patients undergoing

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ﬁ

rst-episode psychosis, showed no signi

ﬁ

cant di

ﬀ

erences between the

patients and the healthy volunteers (Ha

ﬁ

zi et al., 2017). Conversely, in

a PET imaging study, TSPO was markedly elevated in the prefrontal

cortex, anterior cingulate cortex and insula of patients with major

de-pression. These results contradict the previous

ﬁ

ndings in which TSPO

expression was reduced in the platelets and lymphocytes of patients

with major depression (Setiawan et al., 2015). One important limitation

of these aforementioned studies is the absence of an evaluation of TSPO

expression in the lymphocytes, platelets and brain using PET imaging as

shown in

Table 4.

Table 2

TSPO ligands and their therapeutic eﬀects in pre-clinical models of neuropsychiatric disorders.

Experimental study Specie Ligand Dose Outcome Reference

Alzheimer’s disease Mice PK 11195 3 mg/kg once per

week for 4 weeks.

Reduced neuropathology and improved cognitive

behaviour in 3x TgAD mice; reduced solubleβ

-amyloid in wild-type mice.

Barron et al. (2013)

Ro5-4864

Anxiety Rats XBD-173 and

alprazolam

0.1, 1, or 10 mg/kg p.o. and 1 mg/kg i.p. per 5 days.

Both alprazolam and XBD-173 eﬀectively prevented the panic behaviour elicited by an infusion of sodium lactate in panic-prone rats. No sedation was observed after treatment with XBD-173, whereas alprazolam caused a marked reduction in locomotor activity. In the CCK4 challenge paradigm in rats, both alprazolam and XBD-173 clearly displayed anti-panic properties.

Rupprecht et al. (2009)

Brain ischaemia Rats Diazepam 10 mg/kg i.p. 30 and

90 min after the insult.

Provided signiﬁcant neuroprotection: 72.0 ± 14.5% of CA1 hippocampal neurons survived the insult compared to only 12.8 ± 0.3% (n = 15) live neurons in the untreated ischaemic brains. For comparison, the approved neuroprotectant CsA at the optimal dose of 2.5 mg/kg provided a similar protection (61.0 ± 24.2%).

Sarnowska et al. (2009)

Depression Mice ZBD-2 2 or 4 mg/kg, p.o.

once a day for 2 weeks.

Administration of ZBD-2 inhibited decreases in the expression of the synaptic plasticity-related signalling proteins, BDNF and CREB. Moreover, ZBD-2 administration reversed chronic, SCI-induced gliocyte activation at the lesion site.

Li et al. (2017b)

Multiple sclerosis Mice Etifoxine 50 mg/kg for

22 days p.o.

Decreased peripheral immune cell inﬁltration in the spinal cord and increased oligodendroglial regeneration after inﬂammatory demyelination.

Daugherty et al. (2013)

Mice XBD-173 30 mg/kg, single

dose p.o.

In the vehicle group, brain progesterone levels were 6.80 ± 2.86 ng/g, whereas in the XBD-173 treatment group, these levels increased by nearly

six-fold to 39.70 ± 5.01 ng/g (p <0.01).

Allopregnanolone levels were increased by approximately three-fold from 2.70 ± 0.414 ng/g in the vehicle group to 9.41 ± 1.61 ng/g in the

XBD-173 treated animals (p <0.05).

Pregnenolone (p <0.05) and some reduced

metabolites of progesterone, such as 5α-DHP

(p <0.05), 3β5α-THP (p <0.05) and 5α20α

-THP (p <0.01), were also increased by the

XBD-173 treatment.

Ravikumar et al. (2016)

Neuroinﬂammation Mice PK 11195 3 mg/kg, i.p. for

14 days.

Decreasedβ-amyloid1–42content, increased brain

levels of progesterone and allopregnanolone, and protected against cognitive deﬁcits induced by

chronic LPS administration (LPS, 500μg/kg, i.p.

for 11 days).

Ma et al. (2016)

Pneumopathies MRL/lpr mice PK 11195

Ro5-4864

3 mg/kg per i.p. for 30 days.

PK 11195 decreased perivascular inﬁltration, and PK 11195, Ro5-4864, and SSR-180575 decreased peribronchial inﬁltration, and alveolitis in the mouse lungs.

Bribes et al. (2003)

SSR 180575

Post-traumatic stress disorder Mice YL-IPA08 0.1, 0.3, and 1 mg/

kg i.g., for 18 days.

YL-IPA08 caused a suppression of enhanced anxiety and contextual fear induced in the inescapable electric foot-shock-induced mice model of PTSD.

Li et al. (2014)

XBD-173 0.03, 0.1, 0.3, and

1 mg/kg, p.o. for 14 days.

The exposure to foot shocks induced long-term behavioural deﬁciencies in the mice, including freezing and anxiety-like behaviour, which were ameliorated by repeated treatments with XBD-173 but without any eﬀect on spontaneous locomotor activity or body weight.

Rocca et al. (1998)

Rats Midazolam 0.125, 0.25, 0.5, and

1 mg/kg, i.p. administered 30 min before testing.

Midazolam improves the behavioural deﬁcits in the PTSD behaviour in rats as assessed by a single prolonged stress model through dual TSPO and CBR and neurosteroidogenesis

Miao et al., (2014)

Abbreviations: 3xTgAD, triple transgenic Alzheimer’s disease; 3β5α-THP, 3β5α-epiallopregnanolone; 5α-DHP, 5α-dihydroprogesterone; 5α20α-THP, 5α20α-tetrahydroprogesterone; BDNF, brain-derived neurotrophic factor; CA1, regions of the hippocampus; CCK4, cholecystokinin 4; CREB, cyclic AMP-responsive element binding protein; CsA, cyclosporine A; i.g., intragastrointestinally; i.p., intraperitoneally; LPS, lipopolysaccharide; p.o., per orally; PTSD, post-traumatic stress disorder; SCI, spinal cord injury.

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Table 3

TSPO/PET imaging in pre-clinical models of neuropsychiatric disorders.

Experimental study

Specie Outcome Reference

Amyotrophic lateral sclerosis

Mice In the symptomatic SOD1G93A mice, cerebellar, brainstem and cervical spinal cord were increased compared to the values in the WT SOD1 mice, with a statistically significantly difference in the brainstem (p= 0.014). Immunofluorescence studies showed that TSPO [18_{F]DPA-714 expression was increased in the trigeminal, facial, ambiguous and hypoglossal} nuclei, and spinal cord of symptomatic SOD1G93A mice and was localized with increased Iba1 staining.

Gargiulo et al., (2016)

Alzheimer’s disease

Mice Bmaxwas reflective of the number of binding sites and was significantly higher in the frontal cortex of AD brain tissues but did not differ in the cerebellum (p= 0.0002). The binding affinity of [3_{H](R)-PK 11195 re}

ﬂected by the Kd did not diﬀer between the AD and control brain tissues in either brain region (p= 0.3187).

Venneti et al. (2009) Mice [11_{C](R)-PK 11195 distribution volume values in the AD mice were signi}

ﬁcantly higher compared with those in the control mice after a wash-out period of 15 months, which was supported by the immunohistochemistry data (p <0.05). However, [11_{C](R)-PK 11195}_μ_{PET could not demonstrate genotype- or treatment-dependent di}

ﬀerences in the 13- to 14-month-old animals, suggesting that the microglial activation in the AD mice at this age and disease stage is too mild to be detected by this imaging method.

Rapic et al. (2013)

Mice Autoradiography with [3_{H]PBR28 was carried out in the same brains to further con}

ﬁrm the distribution of the radioligand. In addition, immunohistochemistry was performed on adjacent brain sections of the same mice to evaluate the co-localization of TSPO with microglia. PET imaging revealed that the brain uptake of [11_{C]PBR28 in the 5xFAD mice was increased} compared with that in the control mice. Moreover, the binding of [3_{H]PBR28, as measured by autoradiography, was enriched in cortical and hippocampal brain regions, coinciding with} the positive staining of the microglial marker Iba-1 andβ-amyloid deposits in the same areas.

Mirzaei et al. (2016)

Mice PET images showed a signiﬁcantly higher accumulation of [18_{F]PBR06 in the cortex (}_{p <}_{0.005) and hippocampus (}_{p <}_{0.005) of the mice. A signi}

ficant difference in [18_F]PBR06 uptake in mice was visualized and quantified using autoradiography (cortex/striatump <0.05; hippocampus/striatump <0.001). PET results of 15- to 16-month-old mice correlated well with autoradiography and immunostaining.

James et al. (2015)

Brain ischaemia Rats In vivoPET imaging showed a signiﬁcant decrease in [18_{F]DPA-714 uptake 7 days after cerebral ischaemia in rats treated with minocycline compared to that in saline-treated animals} (p <0.05). Minocycline treatment had no eﬀect on the size of the infarcted area.

Martin et al. (2011)

Rats [11_{C]PK 11195 binding was detected at 4 and 7 days in the ipsilateral hemisphere. Two ROIs were de}

fined in the ipsilateral hemisphere corresponding to the region of infarction (ROI-i1) and the surrounding region (ROI-i2). Two ROIs corresponding to the homologous contralateral regions (ROI-c1 and ROI-c2) were also defined. The standard uptake value in the infarcted region (ROI-i1) was significantly higher than that in the control at 4 (p <0.01) and 7 (p <0.001) days after ischaemia. Additionally, standard uptake value showed moderate increases in the peripheral region surrounding infarction (ROI-i2) at 4 (p <0.05) and 7 (p <0.01) days versus the control.

Rojas et al. (2007)

Brain

inﬂammation

Rats Lipopolysaccharide increased [3_{H]PK 11195 binding in the brain, with the largest increases (two- to threefold) in the temporal and entorhinal cortex, hippocampus, and substantia} innominate. A signiﬁcant (> 50%) decrease in [125_{I]iodoMK801 binding was found in the same brain regions.}

Biegon et al. (2002)

Mice A similar [18_{F]FPEB binding was observed in adults that were either prenatally exposed or not exposed to LPS, suggesting that prenatal treatment did not in}

ﬂuence mGluR5 expression in the adult mice. Prenatally LPS-exposed adolescent mice showed a lower level of [18_{F]FPEB binding than adult mice, suggesting an instability or incomplete maturation in the expression of} mGluR5 at the age of PND37.

Arsenault et al. (2015)

Epilepsy Rats Animals with a substantial SE showed a substantial overexpression of TSPOin vitroin relevant brain regions, such as the hippocampus and amygdala (p <0.001), while animals with mild symptoms displayed a smaller increase in TSPO only in the amygdala (p <0.001). TSPO expression was associated with the OX-42 signal but without an obvious cell loss. Similarin vivo[18_{F]PBR 111 increases in Vd}_{and the simpli}

ﬁed ratio were found in key regions, such as the hippocampus (p <0.05) and amygdala (p <0.01).

Dedeurwaerdere et al. (2012)

Meningitis Rabbits PET signal was localized to the tuberculosis lesion (corresponding to the location noted on the post-mortem gross pathological examination) in CNS tuberculosisM. tuberculosisinfected brains compared with the control animals 24 h post-tracer injection (M. tuberculosis-infected SUV mean ± S.E.M = 0.70 ± 0.09, control SUV mean ± S.E.M. = 0.25 ± 0.02;

p= 0.03).

Tucker et al. (2016)

Stroke Mice [18_{F]PBR06 accumulation peaked within the}

first 5 min post injection and then decreased gradually, remaining significantly higher in the infarct than the non-infarct regions. Displacement or pre-blocking with PK 11195 eliminated the difference in [18_{F]PBR 06 uptake between the infarct and non-infarct regions.}

Qiu et al., (2013)

Subarachnoid haemorrhage

Rats High SAH grades were strongly and positively correlated within vivoPET imaging of TSPO using [18_{F]DPA-714 in the cortex and striatum. In addition, a positive correlation was found in} the cortex in TSPO, with densities determined by imaging and auto-radiographic approaches. Qualitative immunoﬂuorescence studies indicated that overexpression of TSPO was linked to astrocytic/microglial activation.

Thomas et al. (2016)

Abbreviations: ACTH, adrenocorticotropic hormone; AD, Alzheimer’s disease; APP/PS1, APPSwe/PSEN1DeltaE9; CNS: central nervous system; Kd, dissociation constant; LPS, lipopolysaccharide; mGluR5, metabotropic glutamatergic receptor; PBR, peripheral-type benzodiazepine receptor; PET, positron emission tomography; PND, postnatal day; ROI-i1/i2, region of infarction 1/2; SOD1G93A, transgenic hemizygous; B6S J L−Tg[SOD1G93A]1Gur/J mice; StAR, steroidogenic acute regulatory protein; SUV, standardized uptake value; TSPO: translocator protein; Vd, volume of distribution; WT SOD1, Transgenic hemizygous B6SJL-Tg(SOD1)2Gur/J mice.

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Table 4

TSPO expression in patients diagnosed with neuropsychiatric disorders.

Pathology Number of patients and controls Sample Changes in the TSPO expression Reference

Amyotrophic lateral sclerosis Patients with ELA (n = 10) healthy controls (n = 10).

In vivobrain imaging

Voxel-wise analysis showed an increased binding in the motor cortices and corticospinal tracts in patients with amyotrophic lateral sclerosis compared to healthy controls (p <0.05). Region of interest analysis revealed increased [11_{C]PBR 28 binding in the precentral gyrus in patients compared to controls (}_{p <}_{0.05). In the patients, these values} were positively correlated with the upper motor neuron burden scores (p <0.05) and negatively correlated with the amyotrophic lateral sclerosis functional rating scale (p <0.05).

Zurcher et al. (2015)

Alzheimer’s disease AD patients (n = 8), multi-infarct dementia (MID) (n = 9), healthy elderly (n = 9), healthy young (n = 9).

Platelets The TSPO of AD patients diﬀered signiﬁcantly from that of elderly healthy controls (p <0.05), but not from that of MID patients.

Bidder et al. (1990)

AD Patients (n = 35), controls (n = 35). Platelets A significant (p <0.01) reduction in the Bmaxmean values was observed in the platelet membranes of DAT patients compared with those of the healthy controls. The Kd values were similar in all subject groups. No significant differences in platelet TSPO density were noted between males and females both in patients and controls.

Bongioanni et al. (1996)

AD Patients (n = 14), controls (n = 8). In vivobrain imaging

Patients had a greater increase in TSPO binding than controls in inferior parietal lobule, precuneus, occipital cortex, hippocampus, entorhinal cortex, and combined middle and inferior temporal cortex (p <0.05). TSPO binding in temporoparietal regions increased from 3.9% to 6.3% per annum in patients, but ranged from−0.5% to 1% per annum in controls. The change in TSPO binding correlated with cognitive worsening on clinical dementia rating scale-sum of boxes and reduced cortical volume.

Kreisl et al. (2016)

AD patients (n = 25), individuals with mild cognitive impairment (n = 11), healthy controls (n = 21).

Using absolute quantitation, we confirmed that TSPO Vdbinding was greater in AD patients than in healthy controls in expected temporoparietal regions and was not significantly different among the 3 groups in the cerebellum. When the cerebellum was used as a pseudo-reference region, the SUVR method detected greater binding in AD patients (p <0.0005) than controls in the same regions as absolute quantification and in 1 additional region, suggesting SUVR may have greater sensitivity.

Lyoo et al. (2015)

AD patients (n = 26), controls (n = 24). In vivobrain imaging

Region-of interest analysis of [11_{C](R)-PK 11195-PET detected signi}

ficant 20−35% increases in microglial activation in frontal, temporal, parietal, occipital and cingulate cortices (p <0.05) of the AD subjects. [11_{C]PIB-PET revealed} significant two-fold increases in amyloid load in these same cortical areas (p <0.0001) and SPM analysis confirmed the localization of these increases to association areas.

Edison et al. (2008)

AD brains (n = 5), control brains (n = 4). Post mortem brain

Both [125_I]des

ﬂuoro-IDAA1106 and [125_{I]desmethoxy-IDAA1106 were e}

ffectively binding to various brain structures. With both radiolabelled compounds, the binding showed regional inhomogeneity and the specific binding values proved to be the highest in the hippocampus, temporal and parietal cortex, the basal ganglia and thalamus in the AD brains. Compared with age-matched control brains, specific binding in several brain structures (temporal and parietal lobes, thalamus and white matter) in Alzheimer brains was significantly higher, indicating that the radioligands can effectively label-activated microglia and the up-regulated TSPO system in AD.

Gulyas et al. (2009)

AD patients (n = 21), controls (n = 21). In vivobrain imaging

In grey matter areas, [18_{F]FEPPA was signi}

ficantly higher in AD compared with healthy control subjects. Voxel-based analyses confirmed significant clusters of neuroinflammation in the frontal, temporal and parietal cortex in patients with AD. In white matter, [18_{F]FEPPA binding was elevated in the posterior limb of the internal capsule, and the} cingulum bundle. Higher neuroinflammation in the parietal cortex (p= 0.005), and posterior limb of the internal capsule (p= 0.001) was associated with poorer visuospatial function. In addition, a higher [18_{F]FEPPA binding in the} posterior limb of the internal capsule was associated with a greater impairment in language ability (p= 0.004).

Suridjan et al. (2015)

Anxiety Patients (n = 14), controls (n = 30). Lymphocytes TSPO mRNA levels of the anxious patients was lower than matched controls (p <0.001). Nudmamud et al. (2000) Outpatients (n = 10), controls (n = 10). Platelets A highly signiﬁcant increase (69%) was observed in the maximal binding capacity of [3_{H]PK11195 after 4 weeks of}

diazepam treatment (p <0.05). One week of diazepam withdrawal resulted m a non-signiﬁcant reduction (10%) of Bmaxcompared to the level achieved during treatment. The post-treatment level was signiﬁcantly higher (42%) than the pre-treatment level (p <0.05).

Weizman et al. (1987)

Autism Men with ASD (n = 20), healthy men controls (n = 20).

The [11_{C](R)-PK11195 binding potential values were signi}

ﬁcantly higher in multiple brain regions in young adults with ASD compared with those of controls (p <0.05). Brain regions with increased binding potentials included the cerebellum, midbrain, pons, fusiform gyro, and the anterior cingulate and orbitofrontal cortices. The most prominent increase was observed in the cerebellum. The pattern of distribution of [11_{C](R)-PK11195 binding potential values in} these brain regions of ASD and control subjects was similar, whereas the magnitude of the [11_{C](R)-PK11195 binding} potential in the ASD group was greater than that of controls in all regions.

Suzuki et al. (2013)

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Table 4(continued)

Pathology Number of patients and controls Sample Changes in the TSPO expression Reference

Bipolar disorder BD with or without separation anxiety disorder (n = 24) and controls (n = 14)

Platelets The lower density was only evident in the subgroup of bipolar patients who also fulﬁlled criteria for adult separation anxiety disorder.

Abelli et al. (2010)

Patients bipolar type II (depressed and euthymic) (n = 12), patients bipolar type I (euthymic) (n = 8), controls healthy (n = 20).

Platelets The results showed that the Bmaxwas 6263 ± 1960 in BD patients and 4855 ± 1621 in the control subjects, statistically higher in thefirst than in the other group (p <0.01). In contrast, the Kd did not show any difference, with the values 5.43 ± 1.53 in the patients and 5.33 ± 1.92 in the control subjects. No difference between patients with bipolar I and bipolar II disorder was detected.

Marazziti et al. (2005)

Patients with BD (n = 14), controls (n = 11).

A signiﬁcantly increased [11_{C](R)-PK11195 binding potential, which is indicative of neuroin}

flammation, was found in the right hippocampus of the patients compared to the healthy controls (p= 0.033). Although the same trend was observed in the left hippocampus, this difference was not statistically significant.

Haarman et al. (2014)

Cirrhosis Cirrhotic patients (n = 8). In vivobrain imaging

There were regional differences in the CBF, with lowest values in the cortex and highest values in the putamen in both groups of subjects (p <0.05), but no significant differences between the groups. There were no significant differences in the Vdof PK 11195 between regions or between the two groups of subjects. Mean values of Vdranged from 1.0 to 1.1 in both groups of subjects.

Iversen et al. (2006)

Creutzfeldt-Jakob Patients gCJD (n = 1), patient sCJD (n = 2), patients vCJD (n = 2), and healthy controls (n = 9).

Post mortem brain tissue

[11_{C](R)-PK11195 PET showed a signi}

ficant TSPO overexpression at the cortical level in the two sCJD patients and thalamic and cerebellar involvement; very limited parieto-occipital activation in the gCJD case; and significant increases at the subcortical level in the thalamus, basal ganglia, and midbrain and in the cerebellum in the vCJD brain. Along with misfolded prion deposits, neuropathology in all patients revealed neuronal loss, spongiosis and astrogliosis, and a diffuse cerebral and cerebellar microgliosis which was particularly dense in thalamic and basal ganglia structures in the vCJD brain.

Iaccarino et al. (2017)

Generalized anxiety disorder Patients/without medication (n = 8), controls (n = 8).

Lymphocytes TSPO mRNA decreased in untreated GAD patients compared to controls (p <0.001). TSPO mRNA levels returned to control levels after treatment and when patients were recovered from anxiety.

Rocca et al. (1998)

Generalized social phobia Patients/without medication (n = 53), controls (n = 53).

Platelets The GSP group was found to have a signiﬁcantly lower TSPO density than the control group (p= 0.00001). Johnson et al. (1998)

Human immunodeﬁciency virus

HIV-seronegative (n = 4), HIV seropositive without encephalitis (n = 5) and HIV with encephalitis (n = 10).

Post mortem brain tissue

Endothelial and smooth muscle cells, subpial glia, intravascular monocytes and ependymal cells were TSPO-positive. In disease states, elevated TSPO was present in parenchymal microglia, macrophages and some hypertrophic astrocytes, but the distribution of TSPO varied depending on the disease, disease stage and proximity to the lesion or relation to infection. Staining with the two antibodies correlated well in white matter, but one antibody also stained cortical neurones. Quantitative analysis demonstrated a signiﬁcant increase in TSPO in the white matter of HIV encephalitis compared with brains without encephalitis. TSPO expression was also increased in simian immunodeﬁciency virus encephalitis.

Cosenza-Nashat et al. (2009)

Huntington’s diseases Patients HD (n = 11), healthy controls (n = 10).

In HD patients, a signiﬁcant increase in striatal [11_{C](R)-PK11195 binding was observed, which signi}

ﬁcantly correlated with disease severity as reﬂected by the striatal reduction in [11_{C]raclopride binding, the Uni}

ﬁed Huntington’s Disease Rating Scale score, and the patients’unstable triplet repeat index. Also detected were signiﬁcant increases in microglia activation in cortical regions including prefrontal cortex and anterior cingulate.

Pavese et al. (2006)

Patients HD (n = 12), controls healthy (n = 12).

Found himself significant differences in mean [11_{C]PK 11195 BPND}_{between premanifest HD gene carriers and healthy} controls (p <0.001). Premanifest HD gene carriers had significant increased mean [11_{C]PK 11195 BPND}_{in caudate} (p= 0.004), putamen (p= 0.004), ventral striatum (p= 0.004) globus pallidus (p <0.001), thalamus (p <0.001), and precentral gyrus (p= 0.015).

Politis et al. (2015)

Major depressive disorder MD patients (n = 14), controls (n = 13). Platelets Platelet TSPO density Bmaxand the dissociation constant of the receptor did not diﬀer in the MD patients compared with normal controls. Furthermore, no correlation was found between the Bmaxvalues and the severity of the depression or the severity of the anxiety.

MD patients (n = 9). Platelets Platelet TSPO density signiﬁcantly diminished (26.7%) following the course of six ECT treatments (pre-treatment versus posttreatment (p <0.05).

Medically healthy subjects with depression (n = 12), healthy control (n = 10).

In the depression group, the injected dose was 611 ± 133 MBq and the injected mass was 0.055 ± 0.032μg/kg. In the control group the injected dose was 667 ± 98 MBq and the injected mass was 0.069 ± 0.035μg/kg. [11_{C]PBR28 was} rapidly metabolized in plasma in both groups, with an unchanged fraction of 66 ± 20%, 13 ± 6% and 4.6 ± 2.4% at 8, 30, and 90 min post injection for control subjects and 72 ± 17%, 17 ± 6% and 6.1 ± 2.6% at 8, 30 and 90 min post injection for case subjects.

Hannestad et al. (2013)

MD patients (n = 20), healthy control (n = 20).

Including all sub regions of the PFC and several other cortical and subcortical regions indicated a global brain effect of diagnosis with elevated TSPO Vdin the patients with MDE compared with the controls main effect of diagnosis, (p= 0.001). Patients with MDE had significantly greater TSPO Vdin the PFC (p= 0.007), ACC (p= 0.001), and insula (p= 0.001]) compared with healthy controls after controlling for the effect of genotype elevations in magnitude of 26%, 32%, and 33%, respectively.

Setiawan et al. (2015)

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