PET imaging of the translocator protein (TSPO)
The translocator protein 18kDa (TSPO), earlier called peripheral benzodiazepine receptor (PBR) and mitochondrial benzodiazepine receptor, is a five transmembrane domain protein. TSPO is mainly situated in the outer mitochondrial membrane, but it is also present in Golgi apparatus, lysosomes, peroxisomes, nucleus, and on plasma membrane, also in mature human red cells (Batarseh & Papadopoulos, 2010). In mitochondria, TSPO may have role in transporting cholesterol into mitochondria, and TSPO concentrations are high in steroid producing tissues. TSPO ligands modulate mitochondrial and cytosolic Ca2+ dynamics.
Secretory and glandular tissues, such as adrenal glands, pineal gland, salivary glands, and olfactory epithelium contain high levels of TSPO; intermediate levels in renal and myocardial tissues, and low levels in the brain and liver (Gavish et al., 1999; Batarseh & Papadopoulos, 2010). In the brain parenchyma TSPO is located in glial cells (microglia and astrocytes), and has thus been used as a biomarker of activated glial cells (Chen & Guilarte, 2008; Alam et al., 2017; Nomura et al., 2021), but it is present also in the neurovascular unit, including endothelial cells, smooth muscle cells, and red blood cells (Wimberley et al., 2021). Mitochondrial dysfunction or abnormal microglia autophagy may be seen as reduced TSPO expression (Zürcher et al., 2020). Outside of the brain, TSPO is expressed in macrophages, and could be targeted in inflammation imaging.
TSPO is involved in a variety of cellular functions, including cholesterol transport, mitochondrial respiration, apoptosis, cell proliferation and differentiation, and oxidative stress, depending on the tissue (Batarseh & Papadopoulos, 2010). TSPO has been found to be upregulated in certain tumours.
In humans, rs6971 polymorphism in the TSPO gene affects the binding of many TSPO ligands, and cholesterol. It affects the rate of steroid synthesis (Owen et al., 2017), and is associated with bipolar disorder (Colasanti et al., 2013) and plasma cholesterol level (Kim et al., 2018). This genetic polymorphism leads to three TSPO radioligand binding profiles: high-affinity binders (HAB), low-affinity binders (LAB), and mixed-affinity binders (MAB) which express both TSPO types in equal proportion (Owen et al, 2011 and 2012; Kreisl et al., 2010 and 2013a; Yoder et al, 2013). PET radioligands which are less affected by TSPO polymorphism are being developed (Fujita et al., 2017; Kim et al., 2020).
[11C]-(R)-PK11195 is the most used PET tracer for imaging TSPO. Because of the high lipophilicity of PK11195, a relatively high fraction of measured tissue uptake is due to nonspecific binding. Other tracers with better target-to-background ratio have been developed (Dollé et al., 2009; Kreisl et al., 2010; Luus et al., 2010; Boutin et al., 2015; Fujita et al., 2017; Alam et al., 2017; Keller et al., 2017, 2018, and 2019).
Although the PET ligands exhibit saturable binding and reciprocal competition in binding assays, results are not consistent across species (Kreisl et al., 2010; Scarf & Kassiou, 2011). Different PET tracers may bind to heterogeneous sites at TSPO, either overlapping or allosterically coupled, for example caused by TSPO polymerization (Scarf & Kassiou, 2011). The rs6971 polymorphism causes three different brain binding affinity patterns in humans (Owen et al., 2011). [3H]PK11195 was found to bind similarly in brain samples across all patients (Owen et al., 2010), but in heart and lungs differences in PK11195 across patients were seen (Kreisl et al., 2010).
TSPO radioligands are mainly used for imaging neuroinflammation, for instance in MS and AD. [11C]DPA-713 has shown wide-spread glial activation in the brains of patients with post-treatment Lyme disease symptoms (Coughlin et al., 2018), and [11C]PBR28 in patients with fibromyalgia (Albrecht et al., 2019).
TSPO ligands are being studied for imaging of atherosclerotic plaques. [18F]FEMPA uptake is increased in plaques with high macrophage content, but no difference between atherosclerotic and healthy mice was observed (Hellberg et al., 2017).
Quantification of TSPO using PET
Binding of currently available TSPO PET ligands is reversible, and 1- or 2-tissue compartmental models and multiple-time graphical analysis for reversible binding have been used to analyse the PET data. Small animal studies are usually analysed by SUV or brain tissue-to-cerebellum ratio, for instance in the mice study using [18F]GE-180 (López-Picón et al., 2018; Hellberg et al., 2018). In certain animal models of brain diseases, for instance the EAE animal model of MS, the healthy hemisphere can be used as reference region (Airas et al., 2015; Vainio et al., 2019).
In the brain the simplified reference tissue model (SRTM) can be applied, although an ideal reference tissue (containing no TSPO) is not available. A clustering method has been introduced to produce a reference curve for brain [11C]-(R)-PK11195 studies (Turkheimer et al., 2007). Wimberley et al (2021) have reviewed the brain TSPO quantification methods and their limitations.
TSPO level in the normal brain is very low, and thus the blood vessel uptake of TSPO ligands becomes predominant. In AD brain, vascular fibrosis decreases the size of lumens, reduces blood volume, and leads to loss of vascular TSPO, particularly in smooth muscles, as shown by immunohistochemistry (Tomasi et al., 2008). Vascular component can be included in SRTM as a linear term for both the target and reference region, increasing the estimated BPND difference between AD patients and control subjects (Tomasi et al., 2008). In case of [11C]-(R)-PK11195, also the nonspecific binding hinders the visualization of low levels of glial activation.
The abundance of TSPO in peripheral organs affects the availability of PET tracers for binding in the brain (Turkheimer et al., 2007) - displacement studies with unlabelled PK11195 have even resulted in increased uptake of [11C]-(R)-PK11195 in the brain, and blocking studies of [18F]FEPPA and [11C]PBR28 based on SUV have failed because of increased tracer availability with higher mass (Wilson et al., 2008). This emphasizes the need for full quantification with proper input functions (arterial plasma or reference tissue input), instead of semi-quantitative methods like SUV. Assessment of peripheral TSPO level may be less affected by the mass effect: a blocking study in atherosclerosis model in mice demonstrated decreased tissue SUV of [18F]GE-180, even with increased retention of activity in the blood (Hellberg et al., 2018).
With other ligands than PK11195, the variable binding affinity to the three identified human sub-populations (Owen et al., 2011) is the main source of variability in PET results. This has been specifically shown for PBR28 (Owen et al., 2012) and [18F]FEPPA (Mizrahi et al., 2012). Radioligand uptake in LAB group is generally too low to be reliably quantified with PET; therefore genotyping must be done prior to PET to omit the LAB subjects. It has been speculated that non-binders might not have been identified with [11C]-(R)-PK11195 because of its poor specific-to-nonspecific binding ratio (Fujita et al., 2008; Kreisl et al., 2010; Fujita et al., 2017). Of four TSPO tracers [11C]-(R)-PK11195, [11C]PBR28, [18F]DPA-713, and [11C]ER176, the only one that can be used to study all three binding groups is [11C]ER176 (Ikawa et al., 2017; Fujita et al., 2017).
An additional factor which may add to the variance is the binding of TSPO ligands to plasma proteins. PK11195 has been shown to bind strongly to AGP, levels of which vary during infection and inflammatory diseases, and AGP could be locally synthesized at the sites of brain injury (Lockhart et al., 2003).
Detection of mitochondria-rich target tissues
- Quantification [18F]DPA-714 PET studies
- Quantification [11C]PBR28 PET studies
- Quantification of [11C]-(R)-PK11195 PET studies
- PET imaging of inflammation and infection
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Updated at: 2021-06-07
Created at: 2012-09-14
Written by: Vesa Oikonen