Metab Brain Dis (2015) 30:31–45 DOI 10.1007/s11011-014-9633-1

REVIEW ARTICLE

PET and MR imaging of neuroinflammation in hepatic encephalopathy Yun Yan Su & Gui Fen Yang & Guang Ming Lu & Shawn Wu & Long Jiang Zhang

Received: 20 August 2014 / Accepted: 17 November 2014 / Published online: 17 December 2014 # Springer Science+Business Media New York 2014

Abstract Neurological or psychiatric abnormalities associated with hepatic encephalopathy (HE) range from subclinical findings to coma. HE is commonly accompanied with the accumulation of toxic substances in bloodstream. The toxicity effect of hyperammonemia on astrocyte, such as the alteration in neurotransmission, oxidative stress, astrocyte swelling, is considered as an important factor in the pathogenesis of HE. Besides, neuroinflammation has captured more attention in the process of HE, but the mechanism of neuroinflammation leading to HE remains unclear. Molecular imaging techniques such as positron emission tomography (PET) and magnetic resonance imaging (MRI) targeting activated microglia and/ or other mediators appear to be promising noninvasive approaches to assess HE. This review focuses on novel imaging and therapy strategies of neuroinflammation in HE.

Abbreviations HE Hepatic encephalopathy MHE Minimal HE NO Nitric oxide GABA γ-aminobutyric acid ALF Acute liver failure TNF-a Tumor necrosis factor a IL Interleukin iNOS Inducible nitric oxide synthase PET Positrons emission tomography TSPO The translocator protein-18 kDa CBF Cerebral blood flow MRI Magnetic resonance imaging

Introduction Keywords Hepatic encephalopathy . Neuroinflammation . Positron emission tomography . Magnetic resonance imaging . Molecular imaging . Reactive oxygen species (ROS)

Drs. Su and Yang had equal contributions for this work. Y. Y. Su : G. M. Lu : L. J. Zhang (*) Department of Medical Imaging, Jinling Hospital, Medical School of Nanjing University, 305 Zhongshan East Road, Xuanwu District, Nangjing, Jiangsu Province 210002, China e-mail: [email protected] G. F. Yang Department of Nuclear Medicine, Jinling Hospital, Clinical School of the Medical College, Nanjing University, Nanjing, Jiangsu Province 210002, China S. Wu (*) Medical Imaging Institute of Tianjin, Tianjin 300192, China e-mail: [email protected]

Hepatic encephalopathy (HE), a complex neuropsychiatric syndrome, frequently occurs in patients with liver insufficiency and/or portosystemic shunting (Vilstrup et al. 2014). Patients with HE show altered motor function, coordination, psychomotor slowing, sleep-waking cycle, and impaired thought process and cognitive function (Monfort et al. 2009). According to the American Association for the Study of Liver Diseases/European Association for the Study of the Liver (AASLD/EASL), the prevalence of overt HE (OHE) is 10-14 % in patients with cirrhosis, and the incidence of minimal HE (MHE) is 20-80 % (Vilstrup et al. 2014). Currently, the diagnosis of OHE is mainly based on clinical findings of decompensated liver disease (Vilstrup et al. 2014). The diagnosis of MHE is based on a battery of psychometric tests (Ciecko-Michalska et al. 2012; Weissenborn 2013). The psychometric hepatic encephalopathy score (PHES) (Kircheis et al. 2014) and Stroop smart phone application (Stinton and Jayakumar 2013) have been proposed to screen MHE. The

32

Metab Brain Dis (2015) 30:31–45

diagnostic accuracy of psychometric tests is limited (Kircheis et al. 2014). Therefore, it is beneficial to develop qualitative and quantitative methods such as imaging biomarkers to diagnose HE (Zhang et al. 2013). Hyperammonemia has an important role in inducing brain edema and neurological alterations in HE. The mechanism is based on dysfunction of neurotransmission (Monfort et al. 2009; Wright and Jalan 2007) and cerebral energy metabolism (Cordoba 2014). Microglial activation has been shown to play a crucial role in neuroinflammation: Factors such as bloodbrain cytokine transfer and receptor-mediated cytokine signal transduction as well as ammonia, work synergistically to induce astrocyte swelling and brain edema. Molecular imaging, including PET/single-photon emission computed tomography (SPECT) and MRI, has been developed for the detection of neuroinflammation in vivo. Imaging neuroinflammation can help better understand the pathological mechanism behind HE.

Pathological mechanism of HE The pathological mechanism of HE remains unclear. Hyperammonemia is regarded as the main cause of neurological alterations in HE (Table 1). In patients and experimental rodents with HE, the accumulation of ammonia in the brain participates in Glutamine/Glutamate cycle with the help of glutamine synthetase (GS) and/or glutaminase localized almost exclusively in the mitochondria of the astrocytes (Fries et al. 2014; Garcia-Martinez and Cordoba 2011) (Fig. 1). Brain edema is the result of astrocyte swelling due to increasing glutamine rather than blood-brain barrier (BBB) disruption. Cytotoxic edema is the main cause of the brain edema in acute liver failure (ALF) (Ciecko-Michalska et al. 2012; Ott and Vilstrup 2014). In contrast, in chronic liver disease, astrocyte transports myoinositol (mIns) and choline (Cho) outside cell membrane, resulting interstitial brain edema (CieckoMichalska et al. 2012; Scott et al. 2013). Table 1

Hyperammonemia induces neurological impairment by causing disturbances of neurotransmission (Fig. 1). Animal studies revealed that hyperammonemia impaired glutamatenitric oxide (NO)-cGMP pathway via the N-methyl-D-aspartate (NMDA) receptor, thus resulting in learning and memory dysfunction (Ciecko-Michalska et al. 2012; Ott and Vilstrup 2014). Studies have shown that chronic hyperammonemia impairs activation of NO synthase and guanylate cyclase in hippocampus and cerebellum (Monfort et al. 2009). Treating chronic HE or hyperammonemia rats with inhibitors of phosphodiesterase-5 restores cGMP levels in brain as well as the ability to learn a Y maze (Monfort et al. 2009). Hyperammonemia is also associated with synthesis of vesicular γ-aminobutyric acid (GABA) (Ott and Vilstrup 2014). When GABA is released into the synaptic cleft and binds to the GABA-A receptor, an inhibitory signal is elicited (CieckoMichalska et al. 2012; Ott and Vilstrup 2014) (Fig. 1). The inhibitory GABA tone can be aggravated by mitochondrial production of neurosteroid. Neurosteroid may be responsible for modulating GABA-A (Ciecko-Michalska et al. 2012) and/ or NMDA receptors (Gonzalez-Usano et al. 2014). Exogenous pregnenolone sulfate (pregS) has the ability to reduce the extracellular GABA concentration and restore the glutamate-NO-cGMP pathway, subsequently restoring learning ability in hyperammonemic rats (Gonzalez-Usano et al. 2014). Similar treatments have been shown to improve cognitive and motor functions in patients with MHE (GonzalezUsano et al. 2014). Hyperammonemia compromises cerebral energy metabolism: Ammonia in the intramitochondrial is thought to mediate release of reactive oxygen species (ROS)/reactive oxygen and nitric oxide species (RNS) through calcium dependent pathways (Cordoba 2014) (Fig. 1). ROS is involved in degradation of the permeability of the BBB, astrocyte swelling and cerebral edema (Prakash and Mullen 2010). Astrocyte swelling leads to mitochondrial permeability transition (MPT)-that is a sudden opening of the permeability transition pore (PTP) in the inner mitochondrial membrane. MPT can result in swelling of the mitochondrial matrix and defective oxidative

Pathological mechanism of hyperammonemia and neuroinflammation in HE

Main actors

Alterations

Influences

Consequence

Hyperammonemia

Elevated glutamine Impaired NO synthase and of guanylate cyclase Elevated GABA and neurosteroid ROS / RNS and MPT Microglial activation and increased proinflammatory mediators such as TNF, IL-1β and IL-6

Astrocyte swelling Impairing the glutamate-NO-cGMP pathway and the GABA-ergic tone

Brain edema and disturbances of neurotransmission

Neuroinflammation

Damaging cerebral energy metabolism Reducing the glutamate transporters and increasing the activation of mGluR1

NO Nitric oxide, GABA γ-aminobutyric acid, ROS Reactive oxygen species, RNS Reactive nitric oxide species, MPT Mitochondrial permeability transition, TNF Tumor necrosis factor, IL Interleukin, mGluR1 Metabotropic glutamate receptor 1

Metab Brain Dis (2015) 30:31–45

33

Fig. 1 Mechanism of hepatic encephalopathy. In HE, ammonia is accumulated in the brain mainly through the blood-brain barrier (BBB) by passive diffusion. The exceeding ammonia in the brain is removed through the process of glutamine (Gln) synthesis from glutamate (Glu) and ammonia, with the participation of glutamine synthetase (GS) localized almost exclusively in the mitochondria of the astrocytes. Astrocytes provide glutamine to the neuron with the participation of glutaminase. Glutamate can be released by neuron, and retaken by a glutamate receptor in astrocyte, then closes the Gln/Glu cycle. Hyperammonemia decreases the glutamate-NO-cGMP pathway and

increases by modulating GABAA and/or NMDA receptors. Also, ammonia in the intramitochondrial may mediate release of ROS and RNS, accompanied with astrocyte swelling. With the stimulus of ammonia, lactate, glutamate and neurosteroids, a ramified microglia becomes reactive (an amoeboid appearance). The activated microglia and inc rease d production of proinflammat ory cytokines (neuroinflammation) cause the disturbances of neurotransmission. GS Glutamine synthetase, GABA γ-aminobutyric acid, ROS Reactive oxygen species, RNS Reactive nitric oxide species, NMDA N-methyl-Daspartate, TSPO Translocator protein

phosphorylation and adenosine triphosphate (ATP) production. Antioxidants including superoxide dismutase, catalase and vitamin E, are able to inhibit the development of the MPT (Scott et al. 2013). Generally speaking, hyperammonemia induces disturbances of neurotransmission and compromises cerebral energy metabolism through PTP and ROS, leading to neurological impairment. Besides, neuroinflammation has been reported to lead cognitive impairment in rats with HE. The role of the neuroinflammation in HE will be discussed in a later section of this review.

“resting” phenotype). When inflammatory stimulus presents, these cells become reactive, and have an amoeboid appearance (the “activated” phenotype) (Butterworth 2011b; Venneti et al. 2006) (Figs. 1 and 2). The activated microglia migrate towards the lesion site, proliferate and produce neurotoxic factors such as proinflammatory cytokines, modulating tissue damage and neuronal regeneration. In HE, a wide range of molecules can trigger the transformation of microglia from the resting to the active state, such as ammonia, lactate, glutamate, neurosteroid and CCL2 (Butterworth 2011b; McMillin et al. 2014) (Fig. 1). There is increasing evidence that glutamate also promotes microglia response to neuronal injury in an ATP-dependent manner via NMDA-receptor activation and formation of Ca2+-wave (Dou et al. 2012; Sieger et al. 2012).

Neuroinflammation in HE Neuroinflammation plays a crucial role in HE, which is characterized by infiltration of immune cells, activation of glial cells and production of inflammatory mediators. Microglia are the main innate immune cells (David and Kroner 2011). In the absence of an inflammatory stimulus, microglia remain quiescent, and have a ramified appearance (the so-called

Neuroinflammation in ALF The existence of neuroinflammation in HE was first demonstrated in the brain of rats with ALF resulting from hepatic devascularization, which consists of activated microglia together with increased levels of proinflammatory mediators

34

Metab Brain Dis (2015) 30:31–45

Fig. 2 Chronic hyperamonemia or bile-duct ligation (BDL) induce microglia activation, which is reversed by chronic treatment with ibuprofen. Expression of major histocompatibility complex class II (MHCII), a marker of microglia activation, was analyzed in sections from control (C) and hyperammonemic rats (HA) (a) and from BDL rats and sham-operated controls (SM) (b), treated with vehicle (VH) or ibuprofen (IBU). Control rats (C-VH), sham controls (SM-VH), and hyperammonemic (HA-IBU) or BDL (BDL-IBU) rats treated with ibuprofen show typical morphology of resting microglia (ramified).

Untreated hyperammonemic (HA-VH) or BDL (BDL-VH) rats show reactive microglia with ameboid shape. This research was originally published in Gastroenterology. Rodrigo R, Cauli O, Gomez-Pinedo U, Agusti A, Hernandez-Rabaza V, Garcia-Verdugo JM, Felipo V. Hyperammonemia induces neuroinflammation that contributes to cognitive impairment in rats with hepatic encephalopathy. Gastroenterology. 2010;139:675–684. © 2010 AGA Institute Terms and Conditions. With permission

(Butterworth 2011b). Subsequently, many groups have studied neuroinflammation in HE (Table 2). Wright et al. demonstrated that brain produced the proinflammatory cytokines and ammonia in situ (Wright et al. 2007). Microglial activation has also been reported in autopsied brain tissue from a patient with ALF resulting from viral hepatitis (Butterworth 2011a; Butterworth 2013). In experimental animal models of ALF, the intensity of microglial activation and cytokines has been proven to correlate with the severity of HE and presence of brain edema (Bemeur and Butterworth 2013; Butterworth 2011b). IL-1 or TNF receptor gene deletion or minocycline prevented neuroinflammation and delayed the onset and severity of HE and brain edema in the experimental ALF animals (Bemeur and Butterworth 2013; Butterworth 2011b).

Neuroinflammation in chronic liver failure Neuroinflammation also presents in the brain of subjects with chronic liver failure (Table 2). Portacaval shunt (PCS) rats with chronic HE showed strong microglial activation and increased levels of inflammatory markers in the cerebral cortex (Monfort et al. 2009). Treating PCS rats with SB239063, an inhibitor of MAP-kinase-p38, reduced microglial activation and brain inflammatory markers and restored cognitive as well as motor function (Agusti et al. 2014). In rats with bile duct ligation (BDL), strong activation of microglia was demonstrated in cerebellum and mild microglial activation in the CA3 area of the hippocampus, corpus callosum, and piriform cortex, accompanied with the increasing iNOS, IL-1β, and

Metab Brain Dis (2015) 30:31–45 Table 2

Neuroinflammation in acute and chronic liver failure

Acute liver failure

Models of HE

Markers of neuroinflammation

References

Rats with hepatic devascularization

Activation of microglial cells and increased levels of TNF, IL-1β and IL-6 Microglial activation and increasing proinflammatory cytokines

Butterworth 2011b

ALF mouse due to azoxymethane toxic

Patients with acetaminophen overdose Patient with viral hepatitis Chronic liver failure

PCS rats

Proinflammatory cytokines and ammonia in situ Microglial activation

Bemeur and Butterworth 2013; Butterworth 2011b; Butterworth 2013 Wright et al. 2007 Butterworth 2011a; Butterworth 2013

Microglial activation and increased levels of inflammatory markers in the cerebral cortex Activation of microglia and increased levels of iNOS, IL-1β, and PGE2 Microglia activation

Rodrigo et al. 2010

No sign of microglial activation Microglial activation

Butterworth 2011a Zemtsova et al. 2011

MHE patients

Increased IL-6 and IL-18 levels

Monfort et al. 2009; Rodrigo et al. 2010

Cultured microglia

No sign of microglial activation

Zemtsova et al. 2011

BDL rats Hyperammonemia rats model without liver failure Rats with portal vein ligation HE patients

in vitro

35

Agusti et al. 2014; Monfort et al. 2009 Rodrigo et al. 2010

TNF-a Tumor necrosis factor a, IL Interleukin, PCS Portacaval shunt, BDL Bile duct ligation, iNOS Inducible nitric oxide synthase, PGE2 Prostaglandin E2, HE Hepatic encephalopathy, MHE Minimal HE

PGE2 (Rodrigo et al. 2010) (Fig. 2), even without any signs of active infection (Jover et al. 2006). Microglial activation was also present in the cerebral cortex of patients with HE, but not in cirrhotic patients without HE (Zemtsova et al. 2011). Increased interleukin (IL-6 and IL-18) levels in MHE patients correlated with the grade of MHE (Monfort et al. 2009; Rodrigo et al. 2010). Except microglial activation, microglial proliferation was found in patients with chronic alcoholics cirrhosis and HE (Dennis et al. 2014). The microglia in these patients displayed an activated morphology with hypertrophied cell bodies and short, thickened processes. These suggest an early neuroprotective response in HE that ultimately fails to halt the course of the disease because of underlying etiological factors, i.e., hyperammonemia and inflammation (Dennis et al. 2014). Unfortunately, the time course of the event between the progression of HE and the polarization of microglia is not well established, and further studies are needed to uncover the role of microglial proliferation in the development of HE. The mechanism of neuroinflammation in HE Neuroinflammation, as a new component in the pathogenesis of HE, is shown to exacerbate the neuropsychological alterations induced by hyperammonemia (Butterworth 2013) (Fig. 1 and Table 2). Activated microglia have been shown

to modulate the excitatory glutamatergic and inhibitory GABA-ergic synaptic neurotransmission by proinflammatory cytokines (Ji et al. 2014). Rats with HE showed neuroinflammation and neurological alteration that were reversed with anti-inflammatory drugs. In patients with cirrhosis and MHE, the correlation between the levels of these ILs and cognitive impairment has been demonstrated (Monfort et al. 2009). Several experiments in HE have already shown that neuroinflammation correlated with excitatory glutamatergic and inhibitory GABA ergic: Activated microglia were more sensitive in cerebellum than in other brain areas in hyperammonemia and BDL rats (Rodrigo et al. 2010). This regional difference corresponded to the increased GABAergic tone in cerebellum in hyperammonemia rats and the lower extracellular cGMP in the cerebellum in the PCS rats (Monfort et al. 2009; Rodrigo et al. 2010). Taken together, microglial activation might correlate with the increased GABA-ergic tone and the reduced activation of soluble guanylate cyclase by NO. Neuroinflammation has been shown to modulate the expression of glial glutamate transporters in HE. The reduction of glutamate transporters EAAC1 and GLT-1 is accompanied by increased extracellular glutamate in substantia nigra pars reticulata (SNr) in PCS rats, and leads to increased activation of metabotropic glutamate receptor1 (mGluR1). Increased mGluR1 activation in SNr leads to increased GABA in

36

ventromedial thalamus, which in turn reduces glutamate in primary motor cortex, and mediates hypolocomotion in PCS rats (Cauli et al. 2009). Blocking mGluR1 in SNr with a specific antagonist can normalize GABA levels in ventromedial thalamus and reduce extracellular glutamate levels in PCS rats. Treating these PCS rats with ibuprofen can regain the amount of EAAC1 and GLT-1 and restore hypokinesia (Cauli et al. 2009) and learning ability (Rodrigo et al. 2010). These results indicate that neuroinflammation plays an important role on the impairment of the glutamate-NO-cGMP pathway and the GABA-ergic tone in the progress of HE (Fig. 1). One recent study found that hyperammonemia or inflammation alone did not induce cognitive impairment in MHE patients, but the combination of certain levels of hyperammonemia and inflammation was enough to induce cognitive impairment, even without liver disease (Felipo et al. 2012).

Neuroinflammation imaging for HE PET imaging of neuroinflammation Noninvasive imaging of microglial activation in patients with HE may help assess the role of neuroinflammation in the progression of HE and the efficacy of potential therapies. Recent studies have used PET combining pharmacological ligands, such as the translocator protein (TSPO) and other radioligands, to detect the activated microglia. When neuroinflammation is predominated by the activated microglia in some neurological disorders, such as neurodegenerative diseases, they are mostly studied by TSPO targeting tracers in conjunction with PET or SPECT. Figure 3 displays the radioisotopic imaging targets for neuroinflammation. These nuclear imaging modalities provide functional and molecular information with high sensitivity (Rahmim and Zaidi 2008). The following section will describe these advances according to the molecular targets, tracers (or probes) and potential application of molecular imaging (mainly of PET imaging) in HE. The molecular targets of neuroinflammation imaging Translocator protein (TSPO) TSPO, known as the peripheraltype benzodiazepine receptor, is a protein of the outer mitochondrial membrane associated with a voltage-dependent anion channel and a nucleoside transporter (Venneti et al. 2006). TSPO primarily involves in the transportation of cholesterol, the regulation of cell death, mitochondrial respiration, MPT pore opening, cell growth and proliferation, steroidogenesis, chemotaxis, and cellular immunity (Jacobs et al. 2012; Kannan et al. 2009). The TSPO expression is very low in normal brain but elevated in the injuried brain areas (Ching et al. 2012). There is increasing evidence that TSPO is highly

Metab Brain Dis (2015) 30:31–45

expressed in activated microglia, and its expression is negligible in the astrocytes and neurons (Abourbeh et al. 2012; Cagnin et al. 2006; Lavisse et al. 2012; Mattner et al. 2005), making it an ideal target. Increased TSPO expression in the brain has been observed in animal models of both ALF and CLF, as well as in brain tissue from cirrhotic patients who died of HE (Butterworth 2013). Although the mechanisms responsible for increased TSPO expression in activated microglia remains unknown, TSPO has already been proven to be a fruitful target for imaging activated microglia. Multiple studies have already proven that PET with pharmacological ligands of TSPO can detect the activated microglia in various center nervous system diseases (Venneti et al. 2006). Many 11C-labeled and 18 F-labeled PET radiotracers have been utilized in preclinical animal models and humans (Kannan et al. 2009). Other potential molecular targets Beyond microglia, various candidate targets have been developed for neuroinflammation imaging (Fig. 3), such as metabolites (arachidonic acid (AA), β-glucuronidase), (Antunes et al. 2012; Pichika et al. 2012), vascular permeability changes (BBB integrity), immunecompetent resident cell activation (proinflammatory cytokines and ROS) (Shukuri et al. 2011), circulating cells (endothelial cells), monoamine oxidase (MAO) (Ciarmiello 2011), metabotropic glutamate receptor (mGluR) (Holland et al. 2014) and GABAA receptor (Holland et al. 2014). Table 3 illustrates these potential molecular targets, their radioligands, and currently investigated diseases for imaging neuroinflammation. The molecular tracers of neuroinflammation PK11195 PK11195 [1-(2-chlorophenyl)-N-methyl-N-(1methylpropyl)-3-isoquinoline carboxamide] is a specific ligand for the TSPO and was first reported in 1986 (Venneti et al. 2013). The specific PET tracer with a modality of racemic R, S-[N-methyl-11C]PK11195, has been widely used in PET imaging studies for decades (Venneti et al. 2013). [11C] or [3H] PK11195 has shown various degrees of success in both animal models and human subjects of neurological degeneration and acute injury of the brain (Venneti et al. 2006). [11C]PK11195 PET technique has a potential to monitor neurologic disorders related to neuroinflammation (Table 3) (Jacobs et al. 2012; Ouchi et al. 2009; Rapic et al. 2013; Vasudeva et al. 2014; Venneti et al. 2006). For example, in a migraine model, the binding potential of [11C]PK11195 in the spreading depression generated rats was higher than that in the sham-operated control rats (Cui et al. 2009). However, there are several limitations of [11C]PK11195 PET. The accuracy, reliability, and indications of [11C]-PK11195 for detecting neuroinflammation need further study. One recent study by Bartels et al. found there was no difference of binding potential between Parkinson disease (PD) patients and controls. The

Metab Brain Dis (2015) 30:31–45

37

Fig. 3 Schematic display of radioisotopic imaging targets for neuroinflammation. Counter clockwise starting from upper left and with increasing nearness and relevance to neuroinflammation process: BBB disruption, labeled inflammatory cells, AA metabolism, and mitochondrial PBR/TSPO of activated microglia. EDTA ethylenediaminetetraacetic acid,

HMPAO hexamethylpropyleneamine oxime, NI neuroinflammation. This research was originally published in JNM. Winkeler A, Boisgard R, Martin A, and Tavitian B. Radioisotopic imaging of neuroinflammation. J Nucl Med. 2010; 51:1–4. © by the Society of Nuclear Medicine and Molecular Imaging, Inc. With permission

binding potential and distribution volume were slightly higher after the administration of celecoxib in PD patients (Bartels et al. 2010). [11C]PK11195 has displayed its relatively low brain permeability, low brain plasma protein binding, and poor signal-to-noise ratio, thus, decreasing its sensitivity in detecting microglial activation (Corcia et al. 2012; Venneti et al. 2013). Additionally, its short physical half-life of 20 min and the mutation of the TSPO gene limit the translational utility of [11C]PK11195 as a PET biomarker (Kreisl et al. 2013). Finally, evidence indicated that PK11195 decreases microglial activation, iNOS, IL-1β, IL-6, TNF-a levels and the extent of neuronal damage in quinolinic acid-injected rats (Venneti et al. 2006).

2014). Further studies have proven [18 F]DPA-714 is a good alternative for TSPO imaging. In rats intranasally inoculated with HSV-1, on day six or seven after inoculation, small animal PET studies were performed to compare the utility between [ 18 F]DPA-714 and [ 11C]PK11195. The result showed lower non-specific binding in [18 F]DPA-714 than [11C]PK11195. In the infected brain, the total [18 F]DPA-714 uptake was lower than that of [11C]PK11195, with comparable specific uptake (Doorduin et al. 2009). Furthermore, direct comparison of [11C]PK11195 and [18 F]DPA-714 in the same rat models was conducted to avoid the inter-individual variability. In cerebral ischemia rats sequentially with both tracers within 24 h, the core/contralateral ratio showed a significantly higher signal-to-noise ratio for [18 F]DPA-714 than [11C] PK11195 (Boutin et al. 2013). In a rat model of acute neuroinflammation, [18 F]DPA-714 performed better than [11C] PK11195, with the highest ratio of ipsilateral to contralateral uptake and the highest binding potential (Chauveau et al. 2009). It has been shown that [18 F]DPA-714 PET was useful to in vivo monitor therapy efficacy of neuroprotective/antiinflammatory drugs (Awde et al. 2013; Martin et al. 2011; Ribeiro et al. 2014; Wang et al. 2014). A novel fluorine-18labelled analogue of DPA-714, [18 F]DPA-C5yne, has been proven to have a high affinity, selectivity, and specificity of the binding for TSPO when compared to PK11195, and it remains stable in plasma at 37 °C for at least 90 min. According to the preliminary evaluation of DPA-C5yne, it can be a promising radiotracer for imaging neuroinflammation with PET (Médran-Navarrete et al. 2014).

Other TSPO radioligands Apart from PK11195, another radiotracer for TSPO, DPA-714 labeled with fluorine 18, has been developed. It has a longer half-life than [11C]PK11195 (110 min compared to 20 min). [18 F]DPA-714 PET has a potential to serve as a novel modality for dynamically monitoring the brain inflammatory response and disease progression (Awde et al. 2013; Tang et al. 2012; Wang et al. 2014). [18 F]DPA-714 has recently been used in seven healthy volunteers and showed excellent in vivo stability and biodistribution. Its uptake was reversible and reached a peak within 20 min after bolus injection that slowly decreased thereafter (Arlicot et al. 2012). Subsequently, [18 F]DPA-714 is a potential biomarker to evaluate neuroinflammation in amyotrophic lateral sclerosis (ALS) patients (Corcia et al. 2012) and the early post-stroke period patients (Ribeiro et al.

[18 F]FEAnGA

[11C]ketoprofen methylester, [18 F]desbromo-Dup-697, [11C]celecoxib [11C]Ldeprenyl

AA

β-glucuronidase

COX

MAO-B is found in astrocytes and neurons

Influences the concentration of neurotransmitter amines.

Neurodegenerative disorders

AD, bipolar disorder, stroke, and other diseases AD and Huntington disease.

ALS, stroke, and HSV-1

Ischemic stroke, MS, ALS, HSV-1, AD, and PD

Diseases

(Ciarmiello 2011; Jacobs et al. 2012)

(Shukuri et al. 2011)

(Antunes et al. 2012)

(Pichika et al. 2012)

(Ching et al. 2012; Tiwari et al. 2014)

(Corcia et al. 2012; Doorduin et al. 2009; Ribeiro et al. 2014)

(Jacobs et al. 2012; Ouchi et al. 2009; Rapic et al. 2013; Vasudeva et al. 2014; Venneti et al. 2006)

References

TSPO The translocator protein-18 kDa, MS Multiple sclerosis, ALS Amyotrophic lateral sclerosis, HSV-1 Herpes simplex virus-1, AD Alzheimer’s disease, PD Parkinson’s disease, AA Arachidonic acid, COX Cyclooxygenase, MAO Monoamine oxidase

MAO-B

[11C]DAA1106, [18 F]FEDAA1106, [11C]PBR28, [11C]CLINME, [11C]SSR180575, [11C]AC-5216, [18 F]FEDAC, [11C]MBMP [14C]AA, [1–11C]AA, [19 F]FAA A second messenger during neurotransmission Increased release of β-glucuronidase by activated microglia COX-1 and -2 are prostanoid-synthesizing enzymes

Microglial activation

[11C]PK11195

TSPO

[18 F]DPA-714

Significance

Tracers

Examples of PET strategies for imaging neuroinflammation

Potential targets

Table 3

38 Metab Brain Dis (2015) 30:31–45

Metab Brain Dis (2015) 30:31–45

Numerous PET ligands (Table 3) have been synthesized and have demonstrated a high specificity for TSPO in both animals and human brains (Ching et al. 2012; Tiwari et al. 2014). Comparing these new specific radioligands for TSPO (e.g.: [11C]vinpocetine, [18 F]DPA-714, PBR28, DAA1106) with the classic ([11C]PK11195) TSPO ligands in experiment models and patients with neuroinflammation revealed improved affinity, higher brain uptake, decreased nonspecific uptake, and higher specific binding of the novel compounds (Abourbeh et al. 2012; Gulyas et al. 2012; Venneti et al. 2013). Although many available radioligands for the TSPO have been developed, the greatest challenge is how to translate the basic studies into in vivo PET imaging. TSPO polymorphisms influence the binding of TSPO radioligands on PET imaging. Single nucleotide polymorphism (SNP) in exon 4 of the TSPO gene causes a non-conservative ala147thr (Kreisl et al. 2013). Quantification of binding for all tested secondgeneration TSPO radioligands indicated the prevalence of three resulting combinations, including homozygous highaffinity (HH), homozygous low-affinity (LL) and heterozygotes (HL). The genotype of TSPO may explain the differential affinity in vitro and in vivo methods when using [11C]PK11195 (Kreisl et al. 2013). TSPO genotype also influenced PBR28 binding affinity in 27 healthy volunteers and in postmortem brain from individuals with schizophrenia (Corcia et al. 2012). Similar results were found in other TSPO ligands including [18 F]FEPPA (Kreisl et al. 2013) and [18 F]DPA-714 (Corcia et al. 2012). In future, studies about the affinity of novel TSPO ligands might take account of its genetic polymorphism such as the rs6791 loci. Other potential tracers for neuroinflammation Beyond radioligands binding to TSPO, numerous potential tracers for neuroinflammation imaging are shown in Table 3 and Fig. 3. [18 F]-FAA is proper for imaging up-regulated brain AA metabolism in brain inflammation (Pichika et al. 2012). [18 F]FEAnGA has been developed to detect β-glucuronidase release during neuroinflammation in a rat model of herpes encephalitis. A significantly enhanced distribution volume of the tracer was shown in the brains of HSV-1 infected rats (Antunes et al. 2012). Other tracers such as [11C]ketoprofen methylester targeting COX-1 (Shukuri et al. 2011) and [11C]Ldeprenyl targeting MAO-B (Ciarmiello 2011; Jacobs et al. 2012) have been developed to assess microglial activation. However, a large amount of studies are needed to prove the role of these tracers in detecting neuroinflammation. PET application of neuroinflammation imaging in HE TSPO targeted PET has been widely applied in the detection of neuroinflammation in many neurological diseases, including HE. Cagnin et al. detected the expression of TSPO in the brain by [11C]PK11195 in five MHE patients (Cagnin et al.

39

2006). Significant increases in glial [11C]PK11195 binding were found bilaterally in the pallidum, right putamen and right dorsolateral prefrontal region. Patients with the most severe cognitive impairment had the highest increases in regional [11C]PK11195 binding (Fig. 4) (Cagnin et al. 2006). Grover et al. reported that cerebral microglial activation occurred in 11 patients with chronic hepatitis C virus (HCV) infection. [11C]PK11195 binding potential was significantly increased in the caudate nucleus compared to normal controls (Grover et al. 2012). However, Iversen et al. did not find an increased TSPO in patients with HE. In their study, eight cirrhotic patients with an acute overt HE (mean arterial ammonia 81 μmol/l) and five healthy subjects (22 μmol/l) underwent dynamic [11C]PK11195 and 15O-H2O PET imaging. The regional cerebral blood flow (CBF) (15O-H2O scan) and the volume of distribution of PK11195 ([11C]PK11195 scan) were calculated. Unexpectedly, there were no significant differences in the distribution volume of PK11195 between the regions or between the two groups of subjects (Iversen et al. 2006). The reported difference can be attributed to many aspects. Very small sample size substantially reduced the statistical power in these studies. Thus, more efforts are needed to identify neuroinflammation imaging in HE.

MR imaging of neuroinflammation General application by MR imaging of neuroinflammation in HE MRI has been used for investigating HE because of its advantages of no ionizing radiation, high spatial resolution, and multi-parameter imaging ability. MRI sequences, such as

Fig. 4 MRI and [11C](R)-PK11195 PET images of patients with HE. Transverse T1W MR image and co-registered [11C](R)-PK11195 image overlaid on the MRI show the increases in [11C](R)-PK11195 binding sites localized in the frontal lobe, particularly in the anterior cingulate cortex (CC). This research was originally published in Gut. Cagnin A, Taylor-Robinson SD, Forton DM, Banati RB. In vivo imaging of cerebral “peripheral benzodiazepine binding sites” in patients with hepatic encephalopathy. Gut. 2006; 55: 547–53. © 2006 BMJ Publishing Group & British Society of Gastroenterology. With permission

40

diffusion weighted imaging (DWI), diffusion tensor imaging (DTI), MR spectroscopy (MRS) and functional MRI have the potential to uncover the neuropathological mechanism of HE (Chavarria et al. 2013; Rami et al. 2007; Razek et al. 2014; Zhang et al. 2014). The hypothesis of mild brain edema in chronic HE is supported by DWI, DTI and 1H-MRS studies, while functional MRI studies show sequelae of brain edema (Bladowska et al. 2014; Kale et al. 2006; Lodi et al. 2004; Qi et al. 2012). Recently, functional MRI studies have focused on the analysis of resting-state brain networks. Zhang et al. demonstrated that the impairment in the basal gangliathalamocortical circuit could play an important role in mediating cognitive dysfunction in patients with MHE (Zhang et al. 2012). Gadolinium-diethylenetriaminepentaacetic acid (Gd-DTPA) enhanced MRI can assess the activity and extent of the BBB disruption (Wunder et al. 2009). Interested readers can get additional information from the recently published reviews on MRI findings of HE (Chavarria and Cordoba 2014; Zhang et al. 2014). HE has typical 1H-MRS findings, i.e., lower Cho/Creatine (Cr) and mIns/Cr as well as higher glutamine/glutamate (Glx)/ Cr (Fig. 5) compared with healthy controls (Zhang et al. 2013). mIns and Cr are putative glial markers. Cho is a marker for various cell membrane precursors and breakdown products, as well as myelin metabolism. Increased Cho has been found in the presence of macrophage infiltration of the brain. Increased Glx in HE patients can contribute to the elevated ammonia which is detoxified in astrocytes to glutamine by glutamine synthetase. Glutamine (increased Glx/Cr on 1HMRS) increased intracellular osmolality. To maintain osmotic equilibrium, astrocytes export osmolytes such as mIns and Cho (decreased mIns/Cr and Cho/Cr on 1H-MRS) (Zhang Fig. 5 1H-MRS of right basal ganglia in a 48-year-old male with cirrhosis shows the typical cirrhosis associated findings, i.e., evelated Glx and lower Cho and mIns. NAA N-acetyl aspartic acid, Cr creatine, Cho choline, Ins myo-inositol, Glx glutamine/ glutamate, ppm percent per million

Metab Brain Dis (2015) 30:31–45

et al. 2013). These 1H-MRS findings were seen in all examined brain regions of HE patients compared with controls (Zhang et al. 2010; Zhang et al. 2013). Patients infected with HCV showed elevated Cho, Cr, and mIns compared to healthy controls and those with hepatitis B, which may be due to microglial activation. Those cerebral metabolite abnormalities were independent of HE (Bokemeyer et al. 2011; Chang et al. 2013), indicating neuronal damage was mainly due to the HCV infection (Bladowska et al. 2013; Chang et al. 2013). Potential MR application of neuroinflammation in HE Although 1H-MRS indicates the presence of neuroinflammation in HE patients, no direct MR imaging evidence supports the neuroinflammation concept in HE. Compared with other modalities, MR imaging at molecular or cellular level holds promise for illustrating the complex role of neuroinflammation in the progression in HE, as well as for directly monitoring anti-inflammatory treatment effects. Molecular imaging probes are the core for molecular MRI technique. Of molecular imaging probes, iron oxide based MR contrast agents are widely used in inflammatory diseases (Colombo et al. 2012; Herranz et al. 2011; Rodríguez et al. 2008). The circulating iron oxides can migrate into the diseased nervous system. Iron oxide-laden macrophages become visible on MRI as signal loss on T2- and T2*- weighted imaging, thus it helps to specifically detect neuroinflammation (Jacobs et al. 2012; Seale-Goldsmith and Leary 2009; Stoll and Bendszus 2009; Thorek et al. 2011). Additionally, nanoparticles have been developed to target specific cell types and molecular targets via affinity ligands (McCarthy and Weissleder 2008), thus improving the sensitivity and specificity of detecting

Metab Brain Dis (2015) 30:31–45

neuroinflammation. Iron-oxide contrast agent efficiently conjugates with P- or E-selectin, and vascular cell adhesion molecule 1 (VCAM-1) antibodies that can target inflamed, activated endothelium cells (Deddens et al. 2012; Jin et al. 2009). A novel iron-oxide contrast agent targeting P-selectinMNP-PBP (magnetic nanoparticle-P-selectin binding peptide) has been used to image endothelial activation following cerebral ischemia/reperfusion by subtraction map (Deddens et al. 2012; Jin et al. 2009). Molecular Dextran-coated iron oxide could be used as probes for macrophage scavenger receptor, targeting oxidative stress, and adhesion molecule expression in MRI (Mulder et al. 2014). However, to our knowledge, no related molecular MRI studies on neuroinflammation in HE have been reported, its value is needed to be demonstrated in future studies. Ironoxide loaded MRI might provide a false-positive finding, as T2 signal loss can be mimicked by susceptibility artifacts, flow-related signal loss and calcification (Wu et al. 2010). The positive-contrast and dual-contrast MRI techniques offering a “white-marker” may help improve the specificity of imaging (Jin et al. 2009; Tourdias and Dousset 2013).

Perspectives Therapeutic strategies of neuroinflammatory response in HE are multiple (Butterworth 2011b), including nonabsorbable disaccharides (such as lactulose (Vilstrup et al. 2014)), antibiotics (such as rifaximin (Kimer et al. 2014; Vilstrup et al. 2014)), probiotics or fecal transplantation (Bajaj 2014; Dhiman 2013; Floch 2014; Quigley and Monsour 2013). These treatments may help remove both ammonia and inflammatory cytokines. Neuroinflammation based molecular imaging can be an effective method to both make a definitive diagnosis and evaluate the therapeutic effects of HE. Targeted molecular imaging, with imaging probes to target genes, mRNA, DNA, metabolites, proteins, and proteinprotein interactions, can provide information on disease stage and treatment efficacy (Eckelman et al. 2008). Nuclear medicine can be easily applied to determine receptor binding. For example, the TSPO expression could be assessed by PET to associate it with TNF-α-neutralizing activity and the molecular mechanisms underlying brain injury (Ching et al. 2012; Linnman et al. 2013). Molecular MRI can provide evidence of decreased proinflammatory cytokines such as IL-1β and TNF-α, decreased matrix metalloproteinase-9 (MMP-9) production (Candelario-Jalil et al. 2009) from up-regulation of the aquaporin family of water channel protein (Candelario-Jalil et al. 2009) in the intervention of neuroinflammation. The long-circulating polyethylene glycol (PEG) minocycline

41

liposomes (Hu et al. 2009) and the arginine-glycine-aspartic acid (RGD) peptide that target the cell adhesion molecule, αvβ3 integrin (Cai and Chen 2006; Cao et al. 2007; Niu and Chen 2011; Pichler et al. 2005) are also an opportunity to be explored for molecular MRI in HE. To develop novel therapeutic approaches to treate HE, a closer look at immune regulation in the CNS is useful. In terms of neuroprotection in HE, immune responses can also be helpful in controlling and limiting pathogenic responses (Amor et al. 2014). Activated microglia “Yin” and “Yang” (Czeh et al. 2011) features participate in the degenerative insults (Tilleux and Hermans 2007). Despite the well-known anti-inflammatory properties of regulatory microglia in several neurodegenerative disorders (Ellrichmann et al. 2013; Schwartz et al. 2013), astrocytes also contribute to protect from immune-mediated damage by preferentially inducing apoptosis of infiltrating T cells by Fas-FasL interactions. T cells and macrophages react with myelin antigens to cause an immune response, Ultrasuperparamagnetic iron oxide (USPIO) can be used to image macrophage and T-cell infiltration (Chin et al. 2009; Wunder et al. 2009). Whether these novel molecular imaging approaches will translate into clinical applications on HE patients remains to be demonstrated. Multimodality imaging has the potential advantage in uncovering pathogenesis of HE (Berding et al. 2009). For example, the hybrid PET/MR scanner may help improve the anatomical localization of molecular targets and the binding quantification of molecular tracers (Berding et al. 2009; Ciarmiello 2011). 19 F-MRI has been used for cell tracking in the peripheral nervous system (Weise et al. 2011) and the chronic constriction injury lesions (Vasudeva et al. 2014) in rats. So far, 19 F-MRI has limited sensitivity, which may be overcome by improved imaging hardware, imaging sequences, reconstruction techniques, and label development (Stoll et al. 2012). PET/MRI seems a promising imaging tool to detect neuroinflammation in the future. Lastly, several clinical issues on neuroinflammation in HE remain unresolved. Currently published neuroinflammation imaging studies are preliminary and very small numbers of patients were included. Thus, a large prospective study should be performed in future. The time course between the progressing of HE and the activated microglia needs to be further evaluated. Studies on polarization of microglia and its appropriate balance between different subtypes may help better understand the pathological mechanism of HE and the role of neuroinflammation imaging in HE. This will create a potential therapeutic strategy for HE. As TSPO has many genetic polymorphisms, it is obligatory to develop other PET ligands or/and other targets to accurately quantify the neuroinflammation.

42

Conclusion Clinical molecular imaging of HE by PET and MRI, is under development and still emerging. The goals of molecular imaging in HE are to refine risk assessment, facilitate early diagnosis, aid in the development of personalized therapeutic regimens, and monitor the efficacy of complex therapies. Over-expressed TSPO is an important target for the detection of neuroinflammation in HE. Novel TSPO radiotracers are needed to be pursued for detecting microglial activation. MRI can provide the multi-parameter platform, which can reflect alterations in cerebral metabolism, perfusion and white matter integrity in HE patients. 19 F-MRI or validated tracers in conjunction with other modalities such as DWI and fMRI may provide potential markers of neuroinflammation in HE patients. Molecular imaging using PET or MRI seems to have a bright future in diagnosis and monitoring therapeutic effects in HE. Acknowledgments We gratefully acknowledge grant support from Natural Scientific Foundation of China (30700194, 81171313, 81322020, and 81230032 to L.J.Z.) and Program for New Century Excellent Talents in University (NCET-12-0260 to L.J.Z.).

References Abourbeh G, Thézé B, Maroy R, Dubois A, Brulon V, Fontyn Y, Dollé F, Tavitian B, Boisgard R (2012) Imaging microglial/macrophage activation in spinal cords of experimental autoimmune encephalomyelitis rats by positron emission tomography using the mitochondrial 18 kDa translocator protein radioligand [(1)(8)F]DPA-714. J Neurosci 32(17):5728–5736 Agusti A, Dziedzic JL, Hernandez-Rabaza V, Guilarte TR, Felipo V (2014) Rats with minimal hepatic encephalopathy due to portacaval shunt show differential increase of translocator protein (18 kDa) binding in different brain areas, which is not affected by chronic MAP-kinase p38 inhibition. Metab Brain Dis 29(4):955–963 Amor S, Peferoen LA, Vogel DY, Breur M, van der Valk P, Baker D, van Noort JM (2014) Inflammation in neurodegenerative diseases—an update. Immunology 142(2):151–166 Antunes IF, Doorduin J, Haisma HJ, Elsinga PH, van Waarde A, Willemsen AT, Dierckx RA, de Vries EF (2012) 18 F-FEAnGA for PET of beta-glucuronidase activity in neuroinflammation. J Nucl Med 53(3):451–458 Arlicot N, Vercouillie J, Ribeiro MJ, Tauber C, Venel Y, Baulieu JL, Maia S, Corcia P, Stabin MG, Reynolds A, Kassiou M, Guilloteau D (2012) Initial evaluation in healthy humans of [18 F]DPA-714, a potential PET biomarker for neuroinflammation. Nucl Med Biol 39(4):570–578 Awde AR, Boisgard R, Thézé B, Dubois A, Zheng J, Dollé F, Jacobs AH, Tavitian B, Winkeler A (2013) The translocator protein radioligand 18 F-DPA-714 monitors antitumor effect of erufosine in a rat 9 L intracranial glioma model. J Nucl Med 54(12):2125–2131 Bajaj JS (2014) The role of microbiota in hepatic encephalopathy. Gut Microbes 5(3):397–403 Bartels AL, Willemsen AT, Doorduin J, de Vries EF, Dierckx RA, Leenders KL (2010) [11C]-PK11195 PET: quantification of

Metab Brain Dis (2015) 30:31–45 neuroinflammation and a monitor of anti-inflammatory treatment in Parkinson’s disease? Parkinsonism Relat Disord 16(1):57–59 Bemeur C, Butterworth RF (2013) Liver-brain proinflammatory signalling in acute liver failure: role in the pathogenesis of hepatic encephalopathy and brain edema. Metab Brain Dis 28(2):145–150 Berding GBR, Buchert R, Chierichetti F, Grover VP, Kato A, Keiding S, Taylor-Robinson SD (2009) International Society for hepatic encephalopathy and nitrogen metabolism (ISHEN). Radiotracer imaging studies in hepatic encephalopathy: ISHEN practice guidelines. Liver Int 29(5):621–628 Bladowska JZA, Knysz B, Małyszczak K, Kołtowska A, Szewczyk P, Gąsiorowski J, Furdal M, Sąsiadek MJ (2013) Evaluation of early cerebral metabolic, perfusion and microstructural changes in HCVpositive patients: a pilot study. J Hepatol 59(4):651–657 Bladowska J, Knysz B, Zimny A, Małyszczak K, Kołtowska A, Szewczyk P, Gąsiorowski J, Furdal M, Sąsiadek MJ (2014) Value of perfusion-weighted MR imaging in the assessment of early cerebral alterations in neurologically asymptomatic HIV-1-positive and HCV-positive patients. PLoS ONE 9(7):e102214 Bokemeyer M, Ding XQ, Goldbecker A, Raab P, Heeren M, Arvanitis D, Tillmann HL, Lanfermann H, Weissenborn K (2011) Evidence for neuroinflammation and neuroprotection in HCV infectionassociated encephalopathy. Gut 60(3):370–377 Boutin H, Prenant C, Maroy R, Galea J, Greenhalgh AD, Smigova A, Cawthorne C, Julyan P, Wilkinson SM, Banister SD, Brown G, Herholz K, Kassiou M, Rothwell NJ (2013) [18F]DPA-714: direct comparison with [11C]PK11195 in a model of cerebral ischemia in rats. PLoS ONE 8(2):e56441 Butterworth RF (2011a) Hepatic encephalopathy: a central neuroinflammatory disorder? Hepatology 53(4):1372–1376 Butterworth RF (2011b) Neuroinflammation in acute liver failure: mechanisms and novel therapeutic targets. Neurochem Int 59(6):830–836 Butterworth RF (2013) The liver-brain axis in liver failure: neuroinflammation and encephalopathy. Nat Rev Gastroenterol Hepatol 10(9): 522–528 Cagnin A, Taylor-Robinson SD, Forton DM, Banati RB (2006) In vivo imaging of cerebral “peripheral benzodiazepine binding sites” in patients with hepatic encephalopathy. Gut 55(4):547–553 Cai W, Chen X (2006) Anti-angiogenic cancer therapy based on integrin alphavbeta3 antagonism. Anticancer Agents Med Chem 6(5):407–428 Candelario-Jalil E, Yang Y, Rosenberg GA (2009) Diverse roles of matrix metalloproteinases and tissue inhibitors of metalloproteinases in neuroinflammation and cerebral ischemia. Neuroscience 158(3): 983–994 Cao Q, Cai W, Li ZB, Chen K, He L, Li HC, Hui M, Chen X (2007) PET imaging of acute and chronic inflammation in living mice. Eur J Nucl Med Mol Imaging 34(11):1832–1842 Cauli O, Rodrigo R, Piedrafita B, Llansola M, Mansouri MT, Felipo V (2009) Neuroinflammation contributes to hypokinesia in rats with hepatic encephalopathy: ibuprofen restores its motor activity. J Neurosci Res 87(6):1369–1374 Chang L, Munsaka SM, Kraft-Terry S, Ernst T (2013) Magnetic resonance spectroscopy to assess neuroinflammation and neuropathic pain. J Neuroimmune Pharm 8(3):576–593 Chauveau F, Van Camp N, Dollé F, Kuhnast B, Hinnen F, Damont A, Boutin H, James M, Kassiou M, Tavitian B (2009) Comparative evaluation of the translocator protein radioligands 11C-DPA-713, 18 F-DPA-714, and 11C-PK11195 in a rat model of acute neuroinflammation. J Nucl Med 50(3):468–476 Chavarria L, Cordoba J (2014) Magnetic resonance of the brain in chronic and acute liver failure. Metab Brain Dis 29(4):937–944 Chavarria L, Oria M, Romero-Giménez J, Alonso J, Lope-Piedrafita S, Cordoba J (2013) Brain magnetic resonance in experimental acuteon-chronic liver failure. Liver Int 33(2):294–300 Chin CL, Pai M, Bousquet PF, Schwartz AJ, O’Connor EM, Nelson CM, Hradil VP, Cox BF, McRae BL, Fox GB (2009) Distinct

Metab Brain Dis (2015) 30:31–45 spatiotemporal pattern of CNS lesions revealed by USPIO-enhanced MRI in MOG-induced EAE rats implicates the involvement of spino-olivocerebellar pathways. J Neuroimmunol 211(1–2):49–55 Ching AS, Kuhnast B, Damont A, Roeda D, Tavitian B, Dollé F (2012) Current paradigm of the 18-kDa translocator protein (TSPO) as a molecular target for PET imaging in neuroinflammation and neurodegenerative diseases. Insights Imaging 3(1):111–119 Ciarmiello A (2011) Imaging of neuroinflammation. Eur J Nucl Med Mol Imaging 38(12):2198–2201 Ciecko-Michalska I, Szczepanek M, Slowik A, Mach T (2012) Pathogenesis of hepatic encephalopathy. Gastroenterol Res Pract 2012:642108 Colombo M, Carregal-Romero S, Casula MF, Gutiérrez L, Morales MP, Böhm IB, Heverhagen JT, Prosperi D, Parak WJ (2012) Biological applications of magnetic nanoparticles. Chem Soc Rev 41(11): 4306–4334 Corcia P, Tauber C, Vercoullie J, Arlicot N, Prunier C, Praline J, Nicolas G, Venel Y, Hommet C, Baulieu JL, Cottier JP, Roussel C, Kassiou M, Guilloteau D, Ribeiro MJ (2012) Molecular imaging of microglial activation in amyotrophic lateral sclerosis. PLoS ONE 7(12):e52941 Cordoba J (2014) Hepatic encephalopathy: from the pathogenesis to the new treatments. ISRN Hepatol 2014:236268. doi:10.1155/2014/ 236268 Cui Y, Takashima T, Takashima-Hirano M, Wada Y, Shukuri M, Tamura Y, Doi H, Onoe H, Kataoka Y, Watanabe Y (2009) 11C-PK11195 PET for the in vivo evaluation of neuroinflammation in the rat brain after cortical spreading depression. J Nucl Med 50(11):1904–1911 Czeh M, Gressens P, Kaindl AM (2011) The yin and yang of microglia. Dev Neurosci 33(3–4):199–209 David S, Kroner A (2011) Repertoire of microglial and macrophage responses after spinal cord injury. Nat Rev Neurosci 12(7):388–399 Deddens LH, Van Tilborg GA, Mulder WJ, De Vries HE, Dijkhuizen RM (2012) Imaging neuroinflammation after stroke: current status of cellular and molecular MRI strategies. Cerebrovasc Dis 33(4):392–402 Dennis CV, Sheahan PJ, Graeber MB, Sheedy DL, Kril JJ, Sutherland GT (2014) Microglial proliferation in the brain of chronic alcoholics with hepatic encephalopathy. Metab Brain Dis 29(4):1027–1039 Dhiman RK (2013) Gut microbiota and hepatic encephalopathy. Metab Brain Dis 28(2):321–326 Doorduin J, Klein HC, Dierckx RA, James M, Kassiou M, de Vries EF (2009) [11C]-DPA-713 and [18 F]-DPA-714 as new PET tracers for TSPO: a comparison with [11C]-(R)-PK11195 in a rat model of herpes encephalitis. Mol Imaging Biol 11(6):386–398 Dou Y, Wu HJ, Li HQ, Qin S, Wang YE, Li J, Lou HF, Chen Z, Li XM, Luo QM, Duan S (2012) Microglial migration mediated by ATPinduced ATP release from lysosomes. Cell Res 22(6):1022–1033 Eckelman WC, Reba RC, Kelloff GJ (2008) Targeted imaging: an important biomarker for understanding disease progression in the era of personalized medicine. Drug Discov Today 13(17–18):748–759 Ellrichmann G, Reick C, Saft C, Linker RA (2013) The role of the immune system in Huntington’s disease. Clin Dev Immunol 2013: 541259 Felipo V, Urios A, Montesinos E, Molina I, Garcia-Torres ML, Civera M, Olmo JA, Ortega J, Martinez-Valls J, Serra MA, Cassinello N, Wassel A, Jordá E, Montoliu C (2012) Contribution of hyperammonemia and inflammatory factors to cognitive impairment in minimal hepatic encephalopathy. Metab Brain Dis 27(1):51–58 Floch MH (2014) Intestinal microbiota metabolism of a prebiotic to treat hepatic encephalopathy. Clin Gastroenterol Hepatol doi:10.1016/j. cgh.2014.06.008 Fries AW, Dadsetan S, Keiding S, Bak LK, Schousboe A, Waagepetersen HS, Simonsen M, Ott P, Vilstrup H, Sørensen M (2014) Effect of glutamine synthetase inhibition on brain and interorgan ammonia metabolism in bile duct ligated rats. J Cereb Blood Flow Metab 34(3):460–466

43 Garcia-Martinez R, Cordoba J (2011) Acute-on-chronic liver failure: the brain. Curr Opin Crit Care 17(2):177–183 Gonzalez-Usano A, Cauli O, Agusti A, Felipo V (2014) Pregnenolone sulfate restores the glutamate-nitric-oxide-cGMP pathway and extracellular GABA in cerebellum and learning and motor coordination in hyperammonemic rats. ACS Chem Neurosci 5(2):100–105 Grover VP, Pavese N, Koh SB, Wylezinska M, Saxby BK, Gerhard A, Forton DM, Brooks DJ, Thomas HC, Taylor-Robinson SD (2012) Cerebral microglial activation in patients with hepatitis C: in vivo evidence of neuroinflammation. J Viral Hepat 19(2):e89–e96 Gulyas B, Toth M, Vas A, Shchukin E, Kostulas K, Hillert J, Halldin C (2012) Visualising neuroinflammation in post-stroke patients: a comparative PET study with the TSPO molecular imaging biomarkers [11C]PK11195 and [11C]vinpocetine. Curr Radiopharm 5(1):19–28 Herranz F, Almarza E, Rodríguez I, Salinas B, Rosell Y, Desco M, Bulte JW, Ruiz-Cabello J (2011) The application of nanoparticles in gene therapy and magnetic resonance imaging. Microsc Res Tech 74(7): 577–591 Holland JP, Liang SH, Rotstein BH, Collier TL, Stephenson NA, Greguric I, Vasdev N (2014) Alternative approaches for PET radiotracer development in Alzheimer’s disease: imaging beyond plaque. J Label Compd Radiopharm 57(4):323–331 Hu W, Metselaar J, Ben LH, Cravens PD, Singh MP, Frohman EM, Eagar TN, Racke MK, Kieseier BC, Stüve O (2009) PEG minocyclineliposomes ameliorate CNS autoimmune disease. PLoS ONE 4(1): e4151 Iversen P, Hansen DA, Bender D, Rodell A, Munk OL, Cumming P, Keiding S (2006) Peripheral benzodiazepine receptors in the brain of cirrhosis patients with manifest hepatic encephalopathy. Eur J Nucl Med Mol Imaging 33(7):810–816 Jacobs AH, Tavitian B, Consortium IN (2012) Noninvasive molecular imaging of neuroinflammation. J Cereb Blood Flow Metab 32(7): 1393–1415 Ji RR, Xu ZZ, Gao YJ (2014) Emerging targets in neuroinflammationdriven chronic pain. Nat Rev Drug Discov 13(7):533–548 Jin AYTU, Rushforth D, Filfil R, Kaur J, Ni F, Tomanek B, Barber PA (2009) Magnetic resonance molecular imaging of post-stroke neuroinflammation with a P-selectin targeted iron oxide nanoparticle. Contrast Media Mol Imaging 4(6):305–311 Jover R, Rodrigo R, Felipo V, Insausti R, Sáez-Valero J, García-Ayllón MS, Suárez I, Candela A, Compañ A, Esteban A, Cauli O, Ausó E, Rodríguez E, Gutiérrez A, Girona E, Erceg S, Berbel P, Pérez-Mateo M (2006) Brain edema and inflammatory activation in bile duct ligated rats with diet-induced hyperammonemia: A model of hepatic encephalopathy in cirrhosis. Hepatology 43(6):1257–1266 Kale RA, Gupta RK, Saraswat VA, Hasan KM, Trivedi R, Mishra AM, Ranjan P, Pandey CM, Narayana PA (2006) Demonstration of interstitial cerebral edema with diffusion tensor MR imaging in type C hepatic encephalopathy. Hepatology 43(4):698–706 Kannan S, Balakrishnan B, Muzik O, Romero R, Chugani D (2009) Positron emission tomography imaging of neuroinflammation. J Child Neurol 24(9):1190–1199 Kimer NKA, Møller S, Bendtsen F, Gluud LL (2014) Systematic review with meta-analysis: the effects of rifaximin in hepatic encephalopathy. Aliment Pharmacol Ther 40(2):123–132 Kircheis G, Hilger N, Haussinger D (2014) Value of critical flicker frequency and psychometric hepatic encephalopathy score in diagnosis of low-grade hepatic encephalopathy. Gastroenterology 146(4):961–969 Kreisl WC, Jenko KJ, Hines CS, Lyoo CH, Corona W, Morse CL, Zoghbi SS, Hyde T, Kleinman JE, Pike VW, McMahon FJ, Innis RB, Biomarkers Consortium PET Radioligand Project Team (2013) A genetic polymorphism for translocator protein 18 kDa affects both in vitro and in vivo radioligand binding in human brain to this putative biomarker of neuroinflammation. J Cereb Blood Flow Metab 33(1):53–58

44 Lavisse SGM, Hérard AS, Petit F, Delahaye M, Van Camp N, Ben Haim L, Lebon V, Remy P, Dollé F, Delzescaux T, Bonvento G, Hantraye P, Escartin C (2012) Reactive astrocytes overexpress TSPO and are detected by TSPO positron emission tomography imaging. J Neurosci 32(32):10809–10818 Linnman C, Becerra L, Borsook D (2013) Inflaming the brain: CRPS a model disease to understand neuroimmune interactions in chronic pain. J Neuroimmune Pharm 8(3):547–563 Lodi R, Tonon C, Stracciari A, Weiger M, Camaggi V, Iotti S, Donati G, Guarino M, Bolondi L, Barbiroli B (2004) Diffusion MRI shows increased water apparent diffusion coefficient in the brains of cirrhotics. Neurology 62(5):762–766 Martin A, Boisgard R, Kassiou M, Dolle F, Tavitian B (2011) Reduced PBR/TSPO expression after minocycline treatment in a rat model of focal cerebral ischemia: a PET study using [(18)F]DPA-714. Mol Imaging Biol 13(1):10–15 Mattner F, Katsifis A, Staykova M, Ballantyne P, Willenborg DO (2005) Evaluation of a radiolabelled peripheral benzodiazepine receptor ligand in the central nervous system inflammation of experimental autoimmune encephalomyelitis: a possible probe for imaging multiple sclerosis. Eur J Nucl Med Mol Imaging 32(5):557–563 McCarthy JR, Weissleder R (2008) Multifunctional magnetic nanoparticles for targeted imaging and therapy. Adv Drug Deliv Rev 60(11): 1241–1251 McMillin M, Frampton G, Thompson M, Galindo C, Standeford H, Whittington E, Alpini G, DeMorrow S (2014) Neuronal CCL2 is upregulated during hepatic encephalopathy and contributes to microglia activation and neurological decline. J Neuroinflammation 11(1):121 Médran-Navarrete V, Bernards N, Kuhnast B, Damont A, Pottier G, Peyronneau MA, Kassiou M, Marguet F, Puech F, Boisgard R, Dollé F (2014) [18 F]DPA-C5yne, a novel fluorine-18-labelled analogue of DPA-714: radiosynthesis and preliminary evaluation as a radiotracer for imaging neuroinflammation with PET. J Label Compd Radiopharm 57(6):410–418 Monfort P, Cauli O, Montoliu C, Rodrigo R, Llansola M, Piedrafita B, El Mlili N, Boix J, Agustí A, Felipo V (2009) Mechanisms of cognitive alterations in hyperammonemia and hepatic encephalopathy: therapeutical implications. Neurochem Int 55(1–3):106–112 Mulder WJ, Jaffer FA, Fayad ZA, Nahrendorf M (2014) Imaging and nanomedicine in inflammatory atherosclerosis. Sci Transl Med 6(239):239sr1 Niu G, Chen X (2011) Why integrin as a primary target for imaging and therapy. Theranostics 1:30–47 Ott P, Vilstrup H (2014) Cerebral effects of ammonia in liver disease: current hypotheses. Metab Brain Dis 29(4):901–911 Ouchi Y, Yagi S, Yokokura M, Sakamoto M (2009) Neuroinflammation in the living brain of Parkinson’s disease. Parkinsonism Relat Disord 15(Suppl 3):S200–S204 Pichika R, Taha AY, Gao F, Kotta K, Cheon Y, Chang L, Kiesewetter D, Rapoport SI, Eckelman WC (2012) The synthesis and in vivo pharmacokinetics of fluorinated arachidonic acid: implications for imaging neuroinflammation. J Nucl Med 53(9):1383–1391 Pichler BJ, Kneilling M, Haubner R, Braumüller H, Schwaiger M, Röcken M, Weber WA (2005) Imaging of delayed-type hypersensitivity reaction by PET and 18 F-galacto-RGD. J Nucl Med 46(1):184–189 Prakash R, Mullen KD (2010) Mechanisms, diagnosis and management of hepatic encephalopathy. Nat Rev Gastroenterol Hepatol 7(9): 515–525 Qi R, Xu Q, Zhang LJ, Zhong J, Zheng G, Wu S, Zhang Z, Liao W, Zhong Y, Ni L, Jiao Q, Zhang Z, Liu Y, Lu G (2012) Structural and functional abnormalities of default mode network in minimal hepatic encephalopathy: a study combining DTI and fMRI. PLoS ONE 7(7):e41376 Quigley EM, Monsour HP (2013) The gut microbiota and the liver: implications for clinical practice. Expert Rev Gastroenterol Hepatol 7(8):723–732

Metab Brain Dis (2015) 30:31–45 Rahmim A, Zaidi H (2008) PET versus SPECT: strengths, limitations and challenges. Nucl Med Commun 29(3):193–207 Rami L, Gómez-Ansón B, Bosch B, Sánchez-Valle R, Monte GC, Villar A, Molinuevo JL (2007) Cortical brain metabolism as measured by proton spectroscopy is related to memory performance in patients with amnestic mild cognitive impairment and Alzheimer’s disease. Dement Geriatr Cogn Disord 24(4):274–279 Rapic S, Backes H, Viel T, Kummer MP, Monfared P, Neumaier B, Vollmar S, Hoehn M, Van der Linden A, Heneka MT, Jacobs AH (2013) Imaging microglial activation and glucose consumption in a mouse model of Alzheimer’s disease. Neurobiol Aging 34(1):351–354 Razek AA, Abdalla A, Ezzat A, Megahed A, Barakat T (2014) Minimal hepatic encephalopathy in children with liver cirrhosis: diffusionweighted MR imaging and proton MR spectroscopy of the brain. Neuroradiology 56(10):885–891 Ribeiro MJ, Vercouillie J, Debiais S, Cottier JP, Bonnaud I, Camus V, Banister S, Kassiou M, Arlicot N, Guilloteau D (2014) Could (18) F-DPA-714 PET imaging be interesting to use in the early poststroke period? EJNMMI Res 4:28 Rodrigo R, Cauli O, Gomez-Pinedo U, Agusti A, Hernandez-Rabaza V, Garcia-Verdugo JM, Felipo V (2010) Hyperammonemia induces neuroinflammation that contributes to cognitive impairment in rats with hepatic encephalopathy. Gastroenterology 139(2):675–684 Rodríguez I, Pérez-Rial S, González-Jimenez J, Pérez-Sánchez J, Herranz F, Beckmann N, Ruíz-Cabello J (2008) Magnetic resonance methods and applications in pharmaceutical research. J Pharm Sci 97(9):3637–3665 Schwartz M, Kipnis J, Rivest S, Prat A (2013) How do immune cells support and shape the brain in health, disease, and aging? J Neurosci 33(45):17587–17596 Scott TR, Kronsten VT, Hughes RD, Shawcross DL (2013) Pathophysiology of cerebral oedema in acute liver failure. World J Gastroenterol 19(48):9240–9255 Seale-Goldsmith MM, Leary JF (2009) Nanobiosystems. Wiley Interdiscip Rev Nanomed Nanobiotechnol 1(5):553–567 Shukuri M, Takashima-Hirano M, Tokuda K, Takashima T, Matsumura K, Inoue O, Doi H, Suzuki M, Watanabe Y, Onoe H (2011) In vivo expression of cyclooxygenase-1 in activated microglia and macrophages during neuroinflammation visualized by PET with 11Cketoprofen methyl ester. J Nucl Med 52(7):1094–1101 Sieger D, Moritz C, Ziegenhals T, Prykhozhij S, Peri F (2012) Longrange Ca2+ waves transmit brain-damage signals to microglia. Dev Cell 22(6):1138–1148 Stinton LM, Jayakumar S (2013) Minimal hepatic encephalopathy. Can J Gastroenterol 27(10):572–574 Stoll G, Bendszus M (2009) Imaging of inflammation in the peripheral and central nervous system by magnetic resonance imaging. Neuroscience 158(3):1151–1160 Stoll GB-LT, Weise G, Jakob P (2012) Visualization of inflammation using (19) F-magnetic resonance imaging and perfluorocarbons. Wiley Interdiscip Rev Nanomed Nanobiotechnol 4(4):438–447 Tang D, Hight MR, McKinley ET, Fu A, Buck JR, Smith RA, Tantawy MN, Peterson TE, Colvin DC, Ansari MS, Nickels M, Manning HC (2012) Quantitative preclinical imaging of TSPO expression in glioma using N, N-diethyl-2-(2-(4-(2–18 F-fluoroethoxy)phenyl)5,7-dimethylpyrazolo[1,5-a]pyrimi din-3-yl)acetamide. J Nucl Med 53(2):287–294 Thorek DL, Weisshaar CL, Czupryna JC, Winkelstein BA, Tsourkas A (2011) Superparamagnetic iron oxide-enhanced magnetic resonance imaging of neuroinflammation in a rat model of radicular pain. Mol Imaging 10(3):206–214 Tilleux S, Hermans E (2007) Neuroinflammation and regulation of glial glutamate uptake in neurological disorders. J Neurosci Res 85(10): 2059–2070 Tiwari AK, Yui J, Fujinaga M, Kumata K, Shimoda Y, Yamasaki T, Xie L, Hatori A, Maeda J, Nengaki N, Zhang MR (2014)

Metab Brain Dis (2015) 30:31–45 Characterization of a novel acetamidobenzoxazolone-based PET ligand for translocator protein (18 kDa) imaging of neuroinflammation in the brain. J Neurochem 129(4):712–720 Tourdias T, Dousset V (2013) Neuroinflammatory imaging biomarkers: relevance to multiple sclerosis and its therapy. Neurotherapeutics 10(1):111–123 Vasudeva K, Andersen K, Zeyzus-Johns B, Hitchens TK, Patel SK, Balducci A, Janjic JM, Pollock JA (2014) Imaging neuroinflammation in vivo in a neuropathic pain rat model with near-infrared fluorescence and (1)(9)F magnetic resonance. PLoS ONE 9(2):e90589 Venneti S, Lopresti BJ, Wiley CA (2006) The peripheral benzodiazepine receptor (Translocator protein 18 kDa) in microglia: from pathology to imaging. Prog Neurobiol 80(6):308–322 Venneti S, Lopresti BJ, Wiley CA (2013) Molecular imaging of microglia/macrophages in the brain. Glia 61(1):10–23 Vilstrup H, Amodio P, Bajaj J, Cordoba J, Ferenci P, Mullen KD, Weissenborn K, Wong P (2014) Hepatic encephalopathy in chronic liver disease: 2014 practice guideline by the American association for the study of liver diseases and the European association for the study of the liver. Hepatology 60(2):715–735 Wang Y, Yue X, Kiesewetter DO, Niu G, Teng G, Chen X (2014) PET imaging of neuroinflammation in a rat traumatic brain injury model with radiolabeled TSPO ligand DPA-714. Eur J Nucl Med Mol Imaging 41(7):1440–1449 Weise G, Basse-Luesebrink TC, Wessig C, Jakob PM, Stoll G (2011) In vivo imaging of inflammation in the peripheral nervous system by (19)F MRI. Exp Neurol 229(2):494–501 Weissenborn K (2013) Psychometric tests for diagnosing minimal hepatic encephalopathy. Metab Brain Dis 28(2):227–229

45 Wright G, Jalan R (2007) Ammonia and inflammation in the pathogenesis of hepatic encephalopathy: Pandora’s box? Hepatology 46(2): 291–294 Wright G, Shawcross D, Olde Damink SW, Jalan R (2007) Brain cytokine flux in acute liver failure and its relationship with intracranial hypertension. Metab Brain Dis 22(3–4):375–388 Wu S, Zhang L, Zhong J, Zhang Z (2010) Dual contrast magnetic resonance imaging tracking of iron-labeled cells in vivo. Cytotherapy 12(7):859–869 Wunder A, Klohs J, Dirnagl U (2009) Non-invasive visualization of CNS inflammation with nuclear and optical imaging. Neuroscience 158(3):1161–1173 Zemtsova I, Görg B, Keitel V, Bidmon HJ, Schrör K, Häussinger D (2011) Microglia activation in hepatic encephalopathy in rats and humans. Hepatology 54(1):204–215 Zhang LJ, Lu GM, Yin JZ, Qi J (2010) Metabolic changes of anterior cingulate cortex in patients with hepatic cirrhosis: a magnetic resonance spectroscopy study. Hepatol Res 40(8): 777–785 Zhang LJ, Zheng G, Zhang L, Zhong J, Wu S, Qi R, Li Q, Wang L, Lu G (2012) Altered brain functional connectivity in patients with cirrhosis and minimal hepatic encephalopathy: a functional MR imaging study. Radiology 265(2):528–536 Zhang LJ, Zhong J, Lu GM (2013) Multimodality MR imaging findings of low-grade brain edema in hepatic encephalopathy. AJNR Am J Neuroradiol 34(4):707–715 Zhang LJ, Wu S, Ren J, Lu GM (2014) Resting-state functional magnetic resonance imaging in hepatic encephalopathy: current status and perspectives. Metab Brain Dis 29(3):569–582