For those of us interested in molecular imaging of the major neurodegenerative disorders, the last decade has witnessed a marked advancement. From basic research to clinical studies, the field has evolved using multidisciplinary techniques and skills, leading to results of significant clinical interest. Reasoning on the role of inflammation in neurodegenerative disease, the recent paper by Santillo et al. [1] is an example of the breadth and depth of the evolving impact of molecular imaging on the field of neurodegeneration and stimulates a reflection on the state of the art of target-specific positron emission tomography (PET) probes of inflammation for brain imaging.

Inflammation in neurodegenerative disease

Reactive astrogliosis is a ubiquitous hallmark of all central nervous system (CNS) pathologies. Chronic neuroinflammation is associated with a broad range of neurodegenerative diseases, including Alzheimer’s disease (AD), Parkinson’s disease (PD), amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA), Huntington’s disease (HD) and all of the tauopathies [2]. Although the key of molecular and cellular events underlying development of different neurodegenerative disorders is clearly divergent, it should be pointed out that death of neurons depends on activation of resident microglial populations in specific brain regions. Up to now some of the molecular and cellular mechanisms of neurodegeneration have been clearly identified.

Neurodegenerative diseases are often characterized by intraneuronal as well as extracellular accumulation of fibrillary materials. Formation of intracellular inclusion bodies resulting from aberrant protein folding, abnormal protein-protein interactions and/or dysregulation of the ubiquitin-proteasome system (UPS) are thought to play a principal role in neuronal dysfunction and death of neurons that characterizes several common neurodegenerative diseases [3].

Brain-resident macrophages (microglia) activation plays a critical role in normal brain function by mediating innate immune responses and are the primary mediators of neuroinflammatory responses. Astrocytes, highly differentiated cells widely distributed in the entire CNS, contribute to every major aspect of brain development, function and disease such as regulation of cerebral blood flow and maintenance of synaptic function, neuronal metabolism and neurotransmitter synthesis [4]. Along with other glial cells such as oligodendrocytes and microglia, astrocytes respond to all forms of CNS insults such as infection, trauma and ischaemia by a process commonly referred to as reactive astrogliosis, which involves changes in their molecular expression and morphology [2]. The growing body of evidence on the role inflammation in neurodegenerative diseases stimulated the demand for imaging modalities for the evaluation of inflammation in human brain, which is the topic of this editorial “Imaging of neuroinflammation”. Some specific proinflammatory factors have been brought to the attention of scientists as possible targets for molecular imaging probes.

Translocator protein system

Translocator protein (TSPO) previously known as the peripheral benzodiazepine receptor is an 18 kDa protein located on the outer membrane of mitochondria overexpressed by activated microglia. TSPO is directly or indirectly involved in many functions, including regulation of cholesterol transport [5], synthesis of steroid hormones [6] and apoptosis [7].

TSPO is expressed only at low levels in the healthy human brain. Increased expression has been observed using the specific ligand 11C-PK11195, which has been shown to reflect the distribution of activated microglia and used as a marker transition from a resting to an activated state in experimental studies and human brain disease [8]. Increased brain binding of 11C-PK11195 was observed in several acute neurological and neurodegenerative disorders such as stroke-induced brain injury, multiple sclerosis, glioma, refractory epilepsy, AD and HD [8–13]. Increased uptake of 11C-PK11195 was also reported in frontotemporal lobar degeneration [14] and HD [15]. In preclinical HD, regional brain atrophy and increased levels of microglial activation in the associative striatum areas (AST) correlated with HD onset [13]. Early striatal dysfunction and cortical atrophy have been previously widely documented and suggested to mark the zone of onset in preclinical presymptomatic HD [16, 17]. Once extended and confirmed, these data might theoretically provide the starting point for studies validating novel premanifest biomarkers. Recently a new promising radioligand with greater affinity for TSPO than PK11195 has been developed. [18F]FEDAA1106 has been proposed for the clinical evaluation of neuroinflammatory diseases and its biodistribution has been recently published [18].

Monoamine oxidase

Monoamine oxidase is an enzyme located at the outer mitochondrial membrane that catalyses the oxidative deamination of a wide range of monoamines, thereby influencing the concentration of neurotransmitter amines. It occurs in two subtypes, MAO-A and MAO-B, which are different gene products. These two isoenzymes differ in their substrate specificities and in their sensitivities to the inhibitors clorgiline and L-deprenyl [19].

MAO-B is localized predominantly in astrocytes and serotonergic neurons, whereas MAO-A is present in catecholaminergic neurons. MAO-B activity has been described to increase with age [20] and in brains of patients with neurodegenerative disorders such as AD [21], HD [22] and ALS [23] as well as after brain injury [37]. MAO-B is localized mainly in glial cells. Increased MAO-B activity may be due to astrocytic proliferation as well as an increased content of MAO-B in reactive astrocytes [24]. MAO-B levels can be estimated using L-deprenyl-D2 (DED), a selective irreversible MAO-B antagonist, which has been developed as a PET tracer [25]. Recent PET studies reported significant increase in DED binding in patients with moderate to severe AD compared with healthy control subjects [1]. Similar results were reported in other neurodegenerative disorders such as ALS [26] and Creutzfeldt-Jakob disease [27]. These findings suggest that in vivo imaging of activated astrocytes is a promising additional tool useful to assess prognosis and monitoring disease activity and treatment.

Amyloid imaging

Activated microglia are present at sites of aggregated Aβ deposition in the brains of AD subjects, although their precise role in the disease process remains unclear [28]. The chemokine receptor CX3CR1 is selectively expressed in microglia and is thought to modulate their activity. Expression of various chemokines as well as plasma levels of fractalkine was reported to be increased in the aging brain and AD patients [29]. A recent study on a mouse model reported lower brain levels of Aβ40 and Aβ42 and reduced amyloid deposits in CX3CX1-deficient mice [30]. The authors reported that CX3CR1 deletion is associated with increased phagocytic ability, which led to greater amyloid content within microglial phagolysosomes. Pittsburgh compound B [PIB; 2-(4′-methylaminophenyl)-6-hydroxybenzothiazole or 6-OH-BTA-1] binds specifically to fibrils of Aβ40 peptides and is a sensitive marker for Aβ pathology in mild cognitive impairment (MCI) and AD. PIB has provided quantitative information on Aβ burden, leading to new insights into Aβ deposition in neurodegenerative disease in which Aβ may play a role [31]. PET studies have shown not only a significative difference in 11C-PIB retention between age-matched healthy controls and AD patients [31], but also inverse correlations with glucose hypometabolism [31]. Furthermore, comparison of the diagnostic utility of Aβ versus 2-[18F]-fluoro-2-deoxy-D-glucose (18F-FDG) imaging demonstrated that PIB imaging may provide an additional tool for the diagnosis of AD [32]. In dementia with Lewy bodies (DLB) patients show an 11C-PIB retention pattern similar to that observed in AD patients.

Preliminary studies report that cognitively normal elderly subjects show an increased 11C-PIB retention in the prefrontal and posterior cingulate/precuneus regions [31]. This increased retention is reported to be lower than that observed in AD subjects. The different increase in 11C-PIB PET retention reported in normal controls, AD and DLB patients reflects a broad range of Aβ deposition [31] which may be related to the degree of microglia activation and/or CX3CR1 expression [30].

Conclusion

There is growing evidence that brain function is likely to reflect a complex scenario which involves pathways and multifunctional cells whose functions are strictly connected. Therefore, it is likely that not one single target or parameter will be used for assessment of neuroinflammation in the future, but a combination of parameters which will allow one to evaluate the role of inflammation in neurodegenerative disorders in its full complexity. Along with new tracers, the next future will bring us the hybrid PET/MR scanner which will improve the anatomical localization of molecular targets and the binding quantitation of molecular tracers through the use of a scanner native software for the correction of partial volume effects on the simultaneously acquired functional and structural images. Interesting perspectives could be also connected with an evaluation of altered pathophysiological parameters such as blood volume, perfusion and blood-brain barrier (BBB) permeability. In this field, together with a better integration between functional PET and functional MRI (or dynamic CT), a major improvement could derive from a revival of old studies allowing a quantitative BBB permeability measurement using rubidium [33] or 68Ga-ethylenediaminetetraacetic acid (EDTA) whose kinetics is not influenced by cellular compartment [34, 35].

Therefore, considering the availability of new tracers and imaging modalities, nuclear medicine will be able to play a relevant role for early diagnosis, selection of patients for therapies and treatment evaluation in this field [36].