TAAR1 Expression in Human Macrophages and Brain Tissue: A Potential Novel Facet of MS Neuroinflammation

Abstract

TAAR1 is a neuroregulator with emerging evidence suggesting a role in immunomodulation. Multiple sclerosis (MS) is an immune-mediated demyelinating disease of the central nervous system. Here, we investigate TAAR1 expression in human primary monocytes, peripherally-derived macrophages, and MS brain tissue. RT-qPCR was used to assess TAAR1 levels in MS monocytes. Using a previously validated anti-human TAAR1 antibody and fluorescence microscopy, TAAR1 protein was visualized in lipopolysaccharide-stimulated or basal human macrophages, as well as macrophage/microglia populations surrounding, bordering, and within a mixed active/inactive MS lesion. In vivo, TAAR1 mRNA expression was significantly lower in MS monocytes compared to age- and sex-matched healthy controls. In vitro, TAAR1 protein showed a predominant nuclear localization in quiescent/control macrophages with a shift to a diffuse intracellular distribution following lipopolysaccharide-induced activation. In brain tissue, TAAR1 protein was predominantly expressed in macrophages/microglia within the border region of mixed active/inactive MS lesions. Considering that TAAR1-mediated anti-inflammatory effects have been previously reported, decreased mRNA in MS patients suggests possible pathophysiologic relevance. A shift in TAAR1 localization following pro-inflammatory activation suggests its function is altered in pro-inflammatory states, while TAAR1-expressing macrophages/microglia bordering an MS lesion supports TAAR1 as a novel pharmacological target in cells directly implicated in MS neuroinflammation.

Keywords: multiple sclerosis; neuroinflammation; trace amine-associated receptor 1; trace amines.

Figure 1

TAAR1 expression in multiple sclerosis (MS) whole peripheral blood mononuclear cells (PBMCs), CD14+ monocytes, and the respective controls. The ΔΔCT method was used to compare TAAR1 expression to the GAPDH housekeeping gene. (A) Brown-Forsythe test showed significant differences between-group variances (* p = 0.0284, F = 4.284). Significant effects were therefore determined by the Kruskal–Wallis test with between-group comparisons by Dunn’s post-hoc multiple comparison test (* p = 0.0168 for NIND vs. Controls, p = 0.4130 for MS vs. Controls). (B) Two-tailed unpaired t test showed a significant difference between the groups (* p = 0.0424, t = 2.194, df = 17). Blue squares denote individual male participants; red circles denote individual female participants.

Figure 2

TAAR1 localization within basal and LPS-stimulated CD14+ monocyte-derived macrophages from healthy volunteers.

Figure 3

TAAR1 expression profile within and surrounding a mixed active/inactive MS lesion. Bright field images of a mixed active/inactive MS lesion were visualized using H&E (A), LFB (B), and CD68 (C) staining. Primary antibodies for TAAR1 and IBA-1 were used in combination with AlexaFluor™ 594(red) and AlexaFluor™ 647(green) conjugated secondary antibodies to visualize TAAR1 expression and microglia/macrophage localization, respectively (D–I). DAPI staining was used to identify nucleated cells (blue). Co-localization of IBA-1 and TAAR1 can be seen in yellow (D–I). Bright-field images were taken with the Cytation™ 5 Cell Imaging Multi-Mode Reader (BioTek). Fluorescent imaging was conducted with the Zeiss AX10 fluorescent microscope at 10× and 40× magnification and images generated in the built-in Zeiss software; Zen 2 Pro.