Purinergic receptors P2Y12R and P2X7R: potential targets for PET imaging of microglia phenotypes in multiple sclerosis

Abstract

BackgroundMicroglia are major players in the pathogenesis of multiple sclerosis (MS) and may play a dual role in disease progression. The activation status of microglia in vivo is highly dynamic and occurs as a continuum, with the pro-inflammatory and anti-inflammatory phenotypes on either end of this spectrum. Little is known about in vivo dynamics of microglia phenotypes in MS due to the lack of diagnostic tools. Positron emission tomography (PET) imaging is a powerful non-invasive technique that allows real-time imaging of microglia activation phenotypes in the central nervous system, depending on the availability of selective PET tracers. Our objective is to investigate and characterize the expression of the purinergic receptors P2Y12R and P2X7R as potential targets for PET tracer development and subsequent PET imaging in order to evaluate the dynamics of microglia status in vivo.MethodsWe used immunohistochemical analysis to explore the expression of P2Y12R and P2X7R in experimental autoimmune encephalomyelitis (EAE) post-mortem tissues and different stages of well-characterized MS lesions. We evaluated by quantitative real-time polymerase chain reaction the expression of P2Y12R and P2X7R in human polarized microglia, and we performed autoradiography binding assay with radiolabeled P2Y12R and P2X7R antagonists using MS and rat EAE tissues.ResultsHere, we demonstrate that P2X7R is associated with a pro-inflammatory phenotype of human microglia in vitro, and is highly expressed in microglia in MS lesions as well as during the peak of EAE. In contrast, P2Y12R was associated with an anti-inflammatory phenotype in human microglia in vitro and was expressed at lower levels in active inflammatory MS lesions compared to normal-appearing white matter (NAWM) and similarly in EAE, while its expression increased in the remission phase of EAE. Binding of radiolabeled tracers specific for P2Y12R and P2X7R on ex vivo tissues validated the value of these receptors as PET imaging targets for microglia phenotypes in vivo.ConclusionOur results suggest that P2Y12R and P2X7R are excellent targets for PET imaging to discriminate distinct microglia phenotypes in MS. Ultimately, this may provide insight into the role of microglia in disease progression and monitor novel treatment strategies to alter microglia phenotype.Electronic supplementary materialThe online version of this article (10.1186/s12974-017-1034-z) contains supplementary material, which is available to authorized users.

PET Imaging of the P2X7 Ion Channel with a Novel Tracer [18F]JNJ-64413739 in a Rat Model of Neuroinflammation

“…In recent years, a few PET tracer candidates to image P2X7 receptors have been proposed [62], and some were evaluated in preclinical models of neuroinflammation [47,63,64]. For example, [ 11 C]GSK1482160 showed increased tracer uptake in both systemic LPS (5 mg/kg) model in mice [47] and in rats with experimental autoimmune encephalomyelitis (EAE) [63].…”

The P2X7 receptor tracer [11C]SMW139 as an in vivo marker of neuroinflammation in multiple sclerosis: a first-in man study

“…Although in general results have been positive, the use of TSPO as a marker for neuroinflammation has some limitations, such as an intracellular binding site, genetic polymorphisms, and additional binding sites on monocytes and vascular wall endothelium [6,7]. In addition, TSPO does not differentiate between resting state and pro-inflammatory and neuro-protective microglia subtypes [8]. Therefore, new PET tracers are needed, which specifically target proteins that reflect the status of microglial cells.…”

“…Therefore, new PET tracers are needed, which specifically target proteins that reflect the status of microglial cells. Recently, using wellcharacterised post-mortem tissues of patients with MS and activated human microglia, it has been demonstrated that the purinergic P2X 7 receptor is highly expressed on proinflammatory microglia and macrophages, is selectively expressed within MS lesions, and may be involved in the neuroinflammatory cascade [8][9][10]. To a lesser extent, expression of P2X 7 receptor has also been found in grey matter on astrocytes, oligodendrocytes, and neurons [11].…”

“…To a lesser extent, expression of P2X 7 receptor has also been found in grey matter on astrocytes, oligodendrocytes, and neurons [11]. Moreover, it was shown that the radioligand [ 11 C]SMW139 selectively binds to the P2X 7 receptor with high affinity in preclinical in vivo studies and in post-mortem human tissues [8,[12][13][14].…”

P2X7 PET Radioligand 18F-PTTP for Differentiation of Lung Tumor from Inflammation

“…Several radiolabeled P2X7 ligands have been developed and evaluated. Neither 11 C-A740003 (14) nor 18 F-EFB (15) showed appreciable brain uptake; however, 3 H-A740003 succeeded in recognizing the site of inflammation in human postmortem brain sections, as well as rodent brain sections when examined by in vitro autoradiography (16). 11 C-GSK1482160 (17) showed great promise due to its strong P2X7 selectivity, efficient radiosynthesis, and blood-brain barrier permeability.…”