GPCRomics of Homeostatic and Disease-Associated Human Microglia

Generation of a Deep Mouse Brain Spectral Library for Transmembrane Proteome Profiling in Mental Disease Models

Transmembrane (TM) proteins constitute over 30% of the mammalian proteome and play essential roles in mediating cell-cell communication, synaptic transmission, and plasticity in the central nervous system. Many of these proteins, especially the G protein-coupled receptors (GPCRs), are validated or candidate drug targets for therapeutic development for mental diseases, yet their expression profiles are underrepresented in most global proteomic studies. Herein, we establish a brain TM protein-enriched spectral library based on 136 data-dependent acquisition runs acquired from various brain regions of both naïve mice and mental disease models. This spectral library comprises 3043 TM proteins including 171 GPCRs, 231 ion channels, and 598 transporters. Leveraging this library, we analyzed the data-independent acquisition data from different brain regions of two mouse models exhibiting depression- or anxiety-like behaviors. By integrating multiple informatics workflows and library sources, our study significantly expanded the mental stress-perturbed TM proteome landscape, from which a new GPCR regulator of depression was verified by in vivo pharmacological testing. In summary, we provide a high-quality mouse brain TM protein spectral library to largely increase the TM proteome coverage in specific brain regions, which would catalyze the discovery of new potential drug targets for the treatment of mental disorders.

Microglia in retinal angiogenesis and diabetic retinopathy

Diabetic retinopathy has a high probability of causing visual impairment or blindness throughout the disease progression and is characterized by the growth of new blood vessels in the retina at an advanced, proliferative stage. Microglia are a resident immune population in the central nervous system, known to play a crucial role in regulating retinal angiogenesis in both physiological and pathological conditions, including diabetic retinopathy. Physiologically, they are located close to blood vessels and are essential for forming new blood vessels (neovascularization). In diabetic retinopathy, microglia become widely activated, showing a distinct polarization phenotype that leads to their accumulation around neovascular tufts. These activated microglia induce pathogenic angiogenesis through the secretion of various angiogenic factors and by regulating the status of endothelial cells. Interestingly, some subtypes of microglia simultaneously promote the regression of neovascularization tufts and normal angiogenesis in neovascularization lesions. Modulating the state of microglial activation to ameliorate neovascularization thus appears as a promising potential therapeutic approach for managing diabetic retinopathy.

Tuning neural circuits and behaviors by microglia in the adult brain

Microglia are the primary immune cells of the CNS, contributing to both inflammatory damage and tissue repair in neurological disorder. In addition, emerging evidence highlights the role of homeostatic microglia in regulating neuronal activity, interacting with synapses, tuning neural circuits, and modulating behaviors. Herein, we review how microglia sense and regulate neuronal activity through synaptic interactions, thereby directly engaging with neural networks and behaviors. We discuss current studies utilizing microglial optogenetic and chemogenetic approaches to modulate adult neural circuits. These manipulations of microglia across different CNS regions lead to diverse behavioral consequences. We propose that spatial heterogeneity of microglia-neuron interaction lays the groundwork for understanding diverse functions of microglia in neural circuits and behaviors.

Chemogenetic activation of microglial Gi signaling decreases microglial surveillance and impairs neuronal synchronization

Microglia actively survey the brain and dynamically interact with neurons to maintain brain homeostasis. Microglial Gi-protein coupled receptors (Gi-GPCRs) play a critical role in microglia-neuron communications. However, the impact of temporally activating microglial Gi signaling on microglial dynamics and neuronal activity in the homeostatic brain remains largely unknown. In this study, we employed Gi-based Designer Receptors Exclusively Activated by Designer Drugs (Gi-DREADD) to selectively and temporally modulate microglial Gi signaling pathway. By integrating this chemogenetic approach with in vivo two-photon imaging, we observed that exogenous activation of microglial Gi signaling transiently inhibited microglial process dynamics, reduced neuronal activity, and impaired neuronal synchronization. These altered neuronal functions were associated with a decrease in interactions between microglia and neuron somata. Altogether, this study demonstrates that acute, exogenous activation of microglial Gi signaling can regulate neuronal circuit function, offering a potential pharmacological target for neuromodulation through microglia.

Aging spinal cord microglia become phenotypically heterogeneous and preferentially target motor neurons and their synapses

Microglia have been found to acquire unique region-dependent deleterious features with age and diseases that contribute to neuronal dysfunction and degeneration in the brain. However, it remains unknown whether microglia exhibit similar phenotypic heterogeneity in the spinal cord. Here, we performed a regional analysis of spinal cord microglia in 3-, 16-, 23-, and 30-month-old mice. Using light and electron microscopy, we discovered that spinal cord microglia acquire an increasingly activated phenotype during the course of aging regardless of regional location. However, aging causes microglia in the ventral but not dorsal horn to lose their spatial organization. Aged ventral horn microglia also aggregate around the somata of motor neurons and increase their contacts with motor synapses, which have been shown to be lost with age. These findings suggest that microglia may affect the ability of motor neurons to receive and relay motor commands during aging. To generate additional insights about aging spinal cord microglia, we performed RNA-sequencing on FACS-isolated microglia from 3-, 18-, 22-, and 29-month-old mice. We found that spinal cord microglia acquire a similar transcriptional identity as those in the brain during aging that includes altered expression of genes with roles in microglia-neuron interactions and inflammation. By 29 months of age, spinal cord microglia exhibit additional and unique transcriptional changes known and predicted to cause senescence and to alter lysosomal and ribosomal regulation. Altogether, this work provides the foundation to target microglia to ameliorate aged-related changes in the spinal cord, and particularly on the motor circuit.

Characterization of cancer-associated fibroblasts (CAFs) and development of a CAF-based risk model for triple-negative breast cancer

Triple-negative breast Cancer (TNBC) is a highly malignant cancer with unclear pathogenesis. Within the tumor microenvironment (TME), cancer-associated fibroblasts (CAFs) vitally influence tumor onset and progression. Thus, this research aimed to identify distinct subgroups of CAF using single-cell and TNBC-related information from the GEO and TCGA databases, respectively. The primary aim was to establish a novel predictive model based on the CAF features and their clinical relevance. Moreover, the CAFs were analyzed for their immune characteristics, response to immunotherapy, and sensitivity to different drugs. The developed predictive model demonstrated significant effectiveness in determining the prognosis of patients with TNBC, TME, and the immune landscape of the tumor. Of note, the expression of GPR34 was significantly higher in TNBC tissues compared to that in other breast cancer (non-TNBC) tissues, indicating that GPR34 plays a crucial role in the onset and progression of TNBC. In summary, this research has yielded a novel predictive model for TNBC that holds promise for the accurate prediction of prognosis and response to immunotherapy in patients with TNBC.

The adhesion G protein-coupled receptor GPR56/ADGRG1 in cytotoxic lymphocytes

GPR56/ADGRG1 is an adhesion G protein-coupled receptor connected to brain development, haematopoiesis, male fertility, and tumorigenesis. Nevertheless, expression of GPR56 is not restricted to developmental processes. Studies over the last years have demonstrated a marked presence of GPR56 in human cytotoxic NK and T cells. Expression of GPR56 in these cells is driven by the transcription factor HOBIT, corresponds with the production of cytolytic mediators and the presence of CX3 CR1 and CD57, indicates a state of terminal differentiation and cellular exhaustion, and disappears upon cellular activation. Functional studies indicate that GPR56 regulates cell migration and effector functions and thereby acts as an inhibitory immune checkpoint. We here discuss the current state of knowledge regarding GPR56 in cytotoxic lymphocytes.

First Potent Macrocyclic A3 Adenosine Receptor Agonists Reveal G-Protein and β-Arrestin2 Signaling Preferences

(N)-Methanocarba adenosine derivatives (A3 adenosine receptor (AR) agonists containing bicyclo[3.1.0]hexane replacing furanose) were chain-extended at N6 and C2 positions with terminal alkenes for ring closure. The resulting macrocycles of 17-20 atoms retained affinity, indicating a spatially proximal orientation of these receptor-bound chains, consistent with molecular modeling of 12. C2-Arylethynyl-linked macrocycle 19 was more A3AR-selective than 2-ether-linked macrocycle 12 (both 5'-methylamides, human (h) A3AR affinities (Ki): 22.1 and 25.8 nM, respectively), with lower mouse A3AR affinities. Functional hA3AR comparison of two sets of open/closed analogues in β-arrestin2 and Gi/o protein assays showed certain signaling preferences divergent from reference agonist Cl-IB-MECA 1. The potencies of 1 at all three Gαi isoforms were slightly less than its hA3AR binding affinity (Ki: 1.4 nM), while the Gαi1 and Gαi2 potencies of macrocycle 12 were roughly an order of magnitude higher than its radioligand binding affinity. Gαi2-coupling was enhanced in macrocycle 12 (EC50 2.56 nM, ∼40% greater maximal efficacy than 1). Di-O-allyl precursor 18 cyclized to form 19, increasing the Gαi1 potency by 7.5-fold. The macrocycles 12 and 19 and their open precursors 11 and 18 potently stimulated β-arrestin2 recruitment, with EC50 values (nM) of 5.17, 4.36, 1.30, and 4.35, respectively, and with nearly 50% greater efficacy compared to 1. This example of macrocyclization altering the coupling pathways of small-molecule (nonpeptide) GPCR agonists is the first for potent and selective macrocyclic AR agonists. These initial macrocyclic derivatives can serve as a guide for the future design of macrocyclic AR agonists displaying unanticipated pharmacology.

Chemogenetic manipulation of CX3CR1+ cells transiently induces hypolocomotion independent of microglia

Chemogenetic approaches using Designer Receptors Exclusively Activated by Designer Drugs (DREADD, a family of engineered GPCRs) were recently employed in microglia. Here, we used Cx3cr1CreER/+:R26hM4Di/+ mice to express Gi-DREADD (hM4Di) on CX3CR1+ cells, comprising microglia and some peripheral immune cells, and found that activation of hM4Di on long-lived CX3CR1+ cells induced hypolocomotion. Unexpectedly, Gi-DREADD-induced hypolocomotion was preserved when microglia were depleted. Consistently, specific activation of microglial hM4Di cannot induce hypolocomotion in Tmem119CreER/+:R26hM4Di/+ mice. Flow cytometric and histological analysis showed hM4Di expression in peripheral immune cells, which may be responsible for the hypolocomotion. Nevertheless, depletion of splenic macrophages, hepatic macrophages, or CD4+ T cells did not affect Gi-DREADD-induced hypolocomotion. Our study demonstrates that rigorous data analysis and interpretation are needed when using Cx3cr1CreER/+ mouse line to manipulate microglia.

The Signaling Pathway of the ADP Receptor P2Y12 in the Immune System: Recent Discoveries and New Challenges

P2Y12 is a G-protein-coupled receptor that is activated upon ADP binding. Considering its well-established role in platelet activation, blocking P2Y12 has been used as a therapeutic strategy for antiplatelet aggregation in cardiovascular disease patients. However, receptor studies have shown that P2Y12 is functionally expressed not only in platelets and the microglia but also in other cells of the immune system, such as in monocytes, dendritic cells, and T lymphocytes. As a result, studies were carried out investigating whether therapies targeting P2Y12 could also ameliorate inflammatory conditions, such as sepsis, rheumatoid arthritis, neuroinflammation, cancer, COVID-19, atherosclerosis, and diabetes-associated inflammation in animal models and human subjects. This review reports what is known about the expression of P2Y12 in the cells of the immune system and the effect of P2Y12 activation and/or inhibition in inflammatory conditions. Lastly, we will discuss the major problems and challenges in studying this receptor and provide insights on how they can be overcome.

Inhibition of Microglial GSK3β Activity Is Common to Different Kinds of Antidepressants: A Proposal for an In Vitro Screen to Detect Novel Antidepressant Principles

Depression is a major public health concern. Unfortunately, the present antidepressants often are insufficiently effective, whilst the discovery of more effective antidepressants has been extremely sluggish. The objective of this review was to combine the literature on depression with the pharmacology of antidepressant compounds, in order to formulate a conceivable pathophysiological process, allowing proposals how to accelerate the discovery process. Risk factors for depression initiate an infection-like inflammation in the brain that involves activation microglial Toll-like receptors and glycogen synthase kinase-3β (GSK3β). GSK3β activity alters the balance between two competing transcription factors, the pro-inflammatory/pro-oxidative transcription factor NFκB and the neuroprotective, anti-inflammatory and anti-oxidative transcription factor NRF2. The antidepressant activity of tricyclic antidepressants is assumed to involve activation of GS-coupled microglial receptors, raising intracellular cAMP levels and activation of protein kinase A (PKA). PKA and similar kinases inhibit the enzyme activity of GSK3β. Experimental antidepressant principles, including cannabinoid receptor-2 activation, opioid μ receptor agonists, 5HT2 agonists, valproate, ketamine and electrical stimulation of the Vagus nerve, all activate microglial pathways that result in GSK3β-inhibition. An in vitro screen for NRF2-activation in microglial cells with TLR-activated GSK3β activity, might therefore lead to the detection of totally novel antidepressant principles with, hopefully, an improved therapeutic efficacy.

3D in vitro modelling of human patient microglia: A focus on clinical translation and drug development in neurodegenerative diseases

Microglia have an increasingly well-recognised role in the pathogenesis of neurodegenerative diseases, thereby becoming attractive therapeutic targets. However, the development of microglia-targeted therapeutics for neurodegeneration has had limited success. This stems partly from the lack of clinically relevant microglia model systems. To circumvent this translational gap, patient-derived microglial cell models established using conventional 2D in vitro techniques have emerged. Though promising, these models lack the microenvironment and multicellular interactions of the brain needed to maintain microglial homeostasis. In this review, we discuss the use of 3D in vitro platforms to improve microglia modelling and their potential benefits to fast-track drug development for neurodegenerative diseases.

Neuroglia Cells Transcriptomic in Brain Development, Aging and Neurodegenerative Diseases

Glia cells are essential for brain functioning during development, aging and disease. However, the role of astroglia plays during brain development is quite different from the role played in the adult lesioned brain. Therefore, a deeper understanding of pathomechanisms underlying astroglia activity in the aging brain and cerebrovascular diseases is essential to guide the development of new therapeutic strategies. To this end, this review provides a comparison between the transcriptomic activity of astroglia cells during development, aging and neurodegenerative diseases, including cerebral ischemia. During fetal brain development, astrocytes and microglia often affect the same developmental processes such as neuro-/gliogenesis, angiogenesis, axonal outgrowth, synaptogenesis, and synaptic pruning. In the adult brain astrocytes are a critical player in the synapse remodeling by mediating synapse elimination while microglia activity has been associated with changes in synaptic plasticity and remove cell debris by constantly sensing the environment. However, in the lesioned brain astrocytes proliferate and play essential functions with regard to energy supply to the neurons, neurotransmission and buildup of a protective scar isolating the lesion site from the surroundings. Inflammation, neurodegeneration, or loss of brain homeostasis induce changes in microglia gene expression, morphology, and function, generally referred to as "primed" microglia. These changes in gene expression are characterized by an enrichment of phagosome, lysosome, and antigen presentation signaling pathways and is associated with an up-regulation of genes encoding cell surface receptors. In addition, primed microglia are characterized by upregulation of a network of genes in response to interferon gamma. Conclusion. A comparison of astroglia cells transcriptomic activity during brain development, aging and neurodegenerative disorders might provide us with new therapeutic strategies with which to protect the aging brain and improve clinical outcome.

Multi-omics reveals Dengzhan Shengmai formulation ameliorates cognitive impairments in D-galactose-induced aging mouse model by regulating CXCL12/CXCR4 and gut microbiota

Dengzhan Shengmai (DZSM), a traditional Chinese medicine formulation, has been administered extensively to elderly individuals with cognitive impairment (CI). However, the underlying mechanisms by which Dengzhan Shengmai improves cognitive impairment remains unknown. This study aimed to elucidate the underlying mechanism of the effect of Dengzhan Shengmai on aging-associated cognitive impairment via a comprehensive combination of transcriptomics and microbiota assessment. Dengzhan Shengmai was orally administered to a D-galactose-induced aging mouse model, and evaluation with an open field task (OFT), Morris water maze (MWM), and histopathological staining was performed. Transcriptomics and 16S rDNA sequencing were applied to elucidate the mechanism of Dengzhan Shengmai in alleviating cognitive deficits, and enzyme-linked immunosorbent assay (ELISA), quantitative real-time polymerase chain reaction (PCR), and immunofluorescence were employed to verify the results. The results first confirmed the therapeutic effects of Dengzhan Shengmai against cognitive defects; specifically, Dengzhan Shengmai improved learning and impairment, suppressed neuro loss, and increased Nissl body morphology repair. Comprehensive integrated transcriptomics and microbiota analysis indicated that chemokine CXC motif receptor 4 (CXCR4) and its ligand CXC chemokine ligand 12 (CXCL12) were targets for improving cognitive impairments with Dengzhan Shengmai and also indirectly suppressed the intestinal flora composition. Furthermore, in vivo results confirmed that Dengzhan Shengmai suppressed the expression of CXC motif receptor 4, CXC chemokine ligand 12, and inflammatory cytokines. This suggested that Dengzhan Shengmai inhibited CXC chemokine ligand 12/CXC motif receptor 4 expression and modulated intestinal microbiome composition by influencing inflammatory factors. Thus, Dengzhan Shengmai improves aging-related cognitive impairment effects via decreased CXC chemokine ligand 12/CXC motif receptor 4 and inflammatory factor modulation to improve gut microbiota composition.

Activation of GPR183 by 7α,25-Dihydroxycholesterol Induces Behavioral Hypersensitivity through Mitogen-Activated Protein Kinase and Nuclear Factor-κB

Emerging evidence implicates the G-protein coupled receptor (GPCR) GPR183 in the development of neuropathic pain. Further investigation of the signaling pathways downstream of GPR183 is needed to support the development of GPR183 antagonists as analgesics. In rodents, intrathecal injection of its ligand, 7α,25-dihydroxycholesterol (7α,25-OHC), causes time-dependent development of mechano-and cold- allodynia (behavioral hypersensitivity). These effects are blocked by the selective small molecule GPR183 antagonist, SAE-14. However, the molecular mechanisms engaged downstream of GPR183 in the spinal cord are not known. Here, we show that 7α,25-OHC-induced behavioral hypersensitivity is Gα i dependent, but not β-arrestin 2-dependent. Non-biased transcriptomic analyses of dorsal-horn spinal cord (DH-SC) tissues harvested at the time of peak hypersensitivity implicate potential contributions of mitogen-activated protein kinase (MAPK) and nuclear factor κB (NF-κB). In support, we found that the development of 7α,25-OHC/GPR183-induced mechano-allodynia was associated with significant activation of MAPKs (extracellular signal-regulated kinase [ERK], p38) and redox-sensitive transcription factors (NF-κB) and increased formation of inflammatory and neuroexcitatory cytokines. SAE-14 blocked these effects and behavioral hypersensitivity. Our findings provide novel mechanistic insight into how GPR183 signaling in the spinal cord produces hypersensitivity through MAPK and NF-κB activation. SIGNIFICANCE STATEMENT: Using a multi-disciplinary approach, we have characterized the molecular mechanisms underpinning 7α,25-OHC/GPR183-induced hypersensitivity in mice. Intrathecal injections of the GPR183 agonist 7α,25-OHC induce behavioral hypersensitivity, and these effects are blocked by the selective GPR183 antagonist SAE-14. We found that 7α,25-OHC-induced allodynia is dependent on MAPK and NF-κB signaling pathways and results in an increase in pro-inflammatory cytokine expression. This study provides a first insight into how GPR183 signaling in the spinal cord is pronociceptive.

Therapeutic non-invasive brain treatments in Alzheimer's disease: recent advances and challenges

Alzheimer's disease (AD) is one of the major neurodegenerative diseases and the most common form of dementia. Characterized by the loss of learning, memory, problem-solving, language, and other thinking abilities, AD exerts a detrimental effect on both patients' and families' quality of life. Although there have been significant advances in understanding the mechanism underlying the pathogenesis and progression of AD, there is no cure for AD. The failure of numerous molecular targeted pharmacologic clinical trials leads to an emerging research shift toward non-invasive therapies, especially multiple targeted non-invasive treatments. In this paper, we reviewed the advances of the most widely studied non-invasive therapies, including photobiomodulation (PBM), transcranial magnetic stimulation (TMS), transcranial direct current stimulation (tDCS), and exercise therapy. Firstly, we reviewed the pathological changes of AD and the challenges for AD studies. We then introduced these non-invasive therapies and discussed the factors that may affect the effects of these therapies. Additionally, we review the effects of these therapies and the possible mechanisms underlying these effects. Finally, we summarized the challenges of the non-invasive treatments in future AD studies and clinical applications. We concluded that it would be critical to understand the exact underlying mechanisms and find the optimal treatment parameters to improve the translational value of these non-invasive therapies. Moreover, the combined use of non-invasive treatments is also a promising research direction for future studies and sheds light on the future treatment or prevention of AD.

Protein Expression of the Microglial Marker Tmem119 Decreases in Association With Morphological Changes and Location in a Mouse Model of Traumatic Brain Injury

The activation of microglia and the infiltration of macrophages are hallmarks of neuroinflammation after acute brain injuries, including traumatic brain injury (TBI). The two myeloid populations share many features in the post-injury inflammatory response, thus, being antigenically indistinguishable. Recently Tmem119, a type I transmembrane protein specifically expressed by microglia under physiological conditions, was proposed as a tool to differentiate resident microglia from blood-borne macrophages, not expressing it. However, the validity of Tmem119 as a specific marker of resident microglia in the context of acute brain injury, where microglia are activated and macrophages are recruited, needs validation. Our purpose was to investigate Tmem119 expression and distribution in relation to the morphology of brain myeloid cells present in the injured area after TBI. Mice underwent sham surgery or TBI by controlled cortical impact (CCI). Brains from sham-operated, or TBI mice, were analyzed by in situ hybridization to identify the cells expressing Tmem119, and by Western blot and quantitative immunofluorescence to measure Tmem119 protein levels in the entire brain regions and single cells. The morphology of Iba1+ myeloid cells was analyzed at different times (4 and 7 days after TBI) and several distances from the contused edge in order to associate Tmem119 expression with morphological evolution of active microglia. In situ hybridization indicated an increased Tmem119 RNA along with increased microglial complement C1q activation in the contused area and surrounding regions. On the contrary, the biochemical evaluation showed a drop in Tmem119 protein levels in the same areas. The Tmem119 immunoreactivity decreased in Iba1+ myeloid cells found in the contused cortex at both time points, with the cells showing the hypertrophic ameboid morphology having no Tmem119 expression. The Tmem119 was present on ramifications of resident microglia and its presence was decreased as a consequence of microglial activation in cortical areas close to contusion. Based on the data, we conclude that the decrease of Tmem119 in reactive microglia may depend on the process of microglial activation, which involves the retracting of their branchings to acquire an ameboid shape. The Tmem119 immunoreactivity decreases in reactive microglia to similar levels than the blood-borne macrophages, thus, failing to discriminate the two myeloid populations after TBI.

In Vitro Oxygen Glucose Deprivation Model of Ischemic Stroke: A Proteomics-Driven Systems Biological Perspective

Oxygen glucose deprivation (OGD) of brain cells is the commonest in vitro model of ischemic stroke that is used extensively for basic and preclinical stroke research. Protein mass spectrometry is one of the most promising and rapidly evolving technologies in biomedical research. A systems-level understanding of cell-type-specific responses to oxygen and glucose deprivation without systemic influence is a prerequisite to delineate the response of the neurovascular unit following ischemic stroke. In this systematic review, we summarize the proteomics studies done on different OGD models. These studies have followed an expression or interaction proteomics approach. They have been primarily used to understand the cellular pathophysiology of ischemia-reperfusion injury or to assess the efficacy of interventions as potential treatment options. We compile the limitations of OGD model and downstream proteomics experiment. We further show that despite having limitations, several proteins shortlisted as altered in in vitro OGD-proteomics studies showed comparable regulation in ischemic stroke patients. This showcases the translational potential of this approach for therapeutic target and biomarker discovery. We next discuss the approaches that can be adopted for cell-type-specific validation of OGD-proteomics results in the future. Finally, we briefly present the research questions that can be addressed by OGD-proteomics studies using emerging techniques of protein mass spectrometry. We have also created a web resource compiling information from OGD-proteomics studies to facilitate data sharing for community usage. This review intends to encourage preclinical stroke community to adopt a hypothesis-free proteomics approach to understand cell-type-specific responses following ischemic stroke.

Adhesion GPCR GPR56 Expression Profiling in Human Tissues

Despite the immense functional relevance of GPR56 (gene ADGRG1) in highly diverse (patho)physiological processes such as tumorigenesis, immune regulation, and brain development, little is known about its exact tissue localization. Here, we validated antibodies for GPR56-specific binding using cells with tagged GPR56 or eliminated ADGRG1 in immunotechniques. Using the most suitable antibody, we then established the human GPR56 tissue expression profile. Overall, ADGRG1 RNA-sequencing data of human tissues and GPR56 protein expression correlate very well. In the adult brain especially, microglia are GPR56-positive. Outside the central nervous system, GPR56 is frequently expressed in cuboidal or highly prismatic secreting epithelia. High ADGRG1 mRNA, present in the thyroid, kidney, and placenta is related to elevated GPR56 in thyrocytes, kidney tubules, and the syncytiotrophoblast, respectively. GPR56 often appears in association with secreted proteins such as pepsinogen A in gastric chief cells and insulin in islet β-cells. In summary, GPR56 shows a broad, not cell-type restricted expression in humans.