Positron Emission Tomography in the Inflamed Cerebellum: Addressing Novel Targets among G Protein-Coupled Receptors and Immune Receptors

Abstract

Inflammatory processes preceding clinical manifestation of brain diseases are moving increasingly into the focus of positron emission tomographic (PET) investigations. A key role in inflammation and as a target of PET imaging efforts is attributed to microglia. Cerebellar microglia, with a predominant ameboid and activated subtype, is of special interest also regarding improved and changing knowledge on functional involvement of the cerebellum in mental activities in addition to its regulatory role in motor function. The present contribution considers small molecule ligands as potential PET tools for the visualization of several receptors recognized to be overexpressed in microglia and which can potentially serve as indicators of inflammatory processes in the cerebellum. The sphingosine 1 phosphate receptor 1 (S1P1), neuropeptide Y receptor 2 (NPY2) and purinoceptor Y12 (P2Y12) cannabinoid receptors and the chemokine receptor CX3CR1 as G-protein-coupled receptors and the ionotropic purinoceptor P2X7 provide structures with rather classical binding behavior, while the immune receptor for advanced glycation end products (RAGE) and triggering receptor expressed on the triggering receptor expressed on myeloid cells 2 (TREM2) might depend for instance on further accessory proteins. Improvement in differentiation between microglial functional subtypes in comparison to the presently used 18 kDa translocator protein ligands as well as of the knowledge on the role of polymorphisms are special challenges in such developments.

Novel plasma protein binding analysis method for a PET tracer and its radiometabolites: A case study with [11C]SMW139 to explain the high uptake of radiometabolites in mouse brain

“…In the brain, P2X 7 receptor is predominantly expressed in microglia and is involved in mediating neuroinflammation [11,12]. Hence, [ 11 C]SMW139 holds broad potential for use in detecting and quantitating neuroinflammation in several neurodegenerative conditions, including Alzheimer's disease, Parkinson's disease, and multiple sclerosis [13,14]. The first radiometabolite study of [ 11 C]SMW139 used a high-performance liquid chromatography system combined with a radiodetector (radioHPLC) method to analyze the parent fraction in rat plasma and brain tissue [9].…”

Targeting TREM2 for Parkinson’s Disease: Where to Go?

“…Increased expression of TREM2 reduced pathological microglial responses and neuropathological and behavioral deficits (127). Endogenous small-molecule inhibitors of TREM2 included phosphatidylethanolamine and phosphatidylserine in vivo (128). In a mouse PD model, TREM2 was also down-regulated by LPS/interleukin-34 (IL-34)/interferon-g (129,130).…”