Gαs-coupled GPCRs GPR65 and GPR174. Downers for immune responses

Lysophosphatidylserine: A Signaling Lipid with Implications in Human Diseases

Lysophosphatidylserine (lyso-PS) has emerged as yet another important signaling lysophospholipid in mammals, and deregulation in its metabolism has been directly linked to an array of human autoimmune and neurological disorders. It has an indispensable role in several biological processes in humans, and therefore, cellular concentrations of lyso-PS are tightly regulated to ensure optimal signaling and functioning in physiological settings. Given its biological importance, the past two decades have seen an explosion in the available literature toward our understanding of diverse aspects of lyso-PS metabolism and signaling and its association with human diseases. In this Review, we aim to comprehensively summarize different aspects of lyso-PS, such as its structure, biodistribution, chemical synthesis, and SAR studies with some synthetic analogs. From a biochemical perspective, we provide an exhaustive coverage of the diverse biological activities modulated by lyso-PSs, such as its metabolism and the receptors that respond to them in humans. We also briefly discuss the human diseases associated with aberrant lyso-PS metabolism and signaling and posit some future directions that may advance our understanding of lyso-PS-mediated mammalian physiology.

Molecular basis of proton-sensing by G protein-coupled receptors

Three proton-sensing G protein-coupled receptors (GPCRs), GPR4, GPR65, and GPR68, respond to changes in extracellular pH to regulate diverse physiology and are implicated in a wide range of diseases. A central challenge in determining how protons activate these receptors is identifying the set of residues that bind protons. Here, we determine structures of each receptor to understand the spatial arrangement of putative proton sensing residues in the active state. With a newly developed deep mutational scanning approach, we determined the functional importance of every residue in proton activation for GPR68 by generating ~9,500 mutants and measuring effects on signaling and surface expression. This unbiased screen revealed that, unlike other proton-sensitive cell surface channels and receptors, no single site is critical for proton recognition in GPR68. Instead, a network of titratable residues extend from the extracellular surface to the transmembrane region and converge on canonical class A GPCR activation motifs to activate proton-sensing GPCRs. More broadly, our approach integrating structure and unbiased functional interrogation defines a new framework for understanding the rich complexity of GPCR signaling.

The epidermal lipid-microbiome loop and immunity: Important players in atopic dermatitis

BACKGROUND

The promotion of epidermal barrier dysfunction is attributed to abnormalities in the lipid-microbiome positive feedback loop which significantly influences the imbalance of the epithelial immune microenvironment (EIME) in atopic dermatitis (AD). This imbalance encompasses impaired lamellar membrane integrity, heightened exposure to epidermal pathogens, and the regulation of innate and adaptive immunity. The lipid-microbiome loop is substantially influenced by intense adaptive immunity which is triggered by abnormal loop activity and affects the loop's integrity through the induction of atypical lipid composition and responses to dysregulated epidermal microbes. Immune responses participate in lipid abnormalities within the EIME by downregulating barrier gene expression and are further cascade-amplified by microbial dysregulation which is instigated by barrier impairment.

AIM OF REVIEW

This review examines the relationship between abnormal lipid composition, microbiome disturbances, and immune responses in AD while progressively substantiating the crosstalk mechanism among these factors. Based on this analysis, the "lipid-microbiome" positive feedback loop, regulated by immune responses, is proposed.

KEY SCIENTIFIC CONCEPTS OF REVIEW

The review delves into the impact of adaptive immune responses that regulate the EIME, driving AD, and investigates potential mechanisms by which lipid supplementation and probiotics may alleviate AD through the up-regulation of the epidermal barrier and modulation of immune signaling. This exploration offers support for targeting the EIME to attenuate AD.

Quinolinate promotes macrophage-induced immune tolerance in glioblastoma through the NMDAR/PPARγ signaling axis

There has been considerable scientific effort dedicated to understanding the biologic consequence and therapeutic implications of aberrant tryptophan metabolism in brain tumors and neurodegenerative diseases. A majority of this work has focused on the upstream metabolism of tryptophan; however, this has resulted in limited clinical application. Using global metabolomic profiling of patient-derived brain tumors, we identify the downstream metabolism of tryptophan and accumulation of quinolinate (QA) as a metabolic node in glioblastoma and demonstrate its critical role in promoting immune tolerance. QA acts as a metabolic checkpoint in glioblastoma by inducing NMDA receptor activation and Foxo1/PPARγ signaling in macrophages, resulting in a tumor supportive phenotype. Using a genetically-engineered mouse model designed to inhibit production of QA, we identify kynureninase as a promising therapeutic target to revert the potent immune suppressive microenvironment in glioblastoma. These findings offer an opportunity to revisit the biologic consequence of this pathway as it relates to oncogenesis and neurodegenerative disease and a framework for developing immune modulatory agents to further clinical gains in these otherwise incurable diseases.

A major mechanism for immunomodulation: Dietary fibres and acid metabolites

Diet and the gut microbiota have a profound influence on physiology and health, however, mechanisms are still emerging. Here we outline several pathways that gut microbiota products, particularly short-chain fatty acids (SCFAs), use to maintain gut and immune homeostasis. Dietary fibre is fermented by the gut microbiota in the colon, and large quantities of SCFAs such as acetate, propionate, and butyrate are produced. Dietary fibre and SCFAs enhance epithelial integrity and thereby limit systemic endotoxemia. Moreover, SCFAs inhibit histone deacetylases (HDAC), and thereby affect gene transcription. SCFAs also bind to 'metabolite-sensing' G-protein coupled receptors (GPCRs) such as GPR43, which promotes immune homeostasis. The enormous amounts of SCFAs produced in the colon are sufficient to lower pH, which affects the function of proton sensors such as GPR65 expressed on the gut epithelium and immune cells. GPR65 is an anti-inflammatory Gα_s-coupled receptor, which leads to the inhibition of inflammatory cytokines. The importance of GPR65 in inflammatory diseases is underscored by genetics associated with the missense variant I231L (rs3742704), which is associated with human inflammatory bowel disease, atopic dermatitis, and asthma. There is enormous scope to manipulate these pathways using specialized diets that release very high amounts of specific SCFAs in the gut, and we believe that therapies that rely on chemically modified foods is a promising approach. Such an approach includes high SCFA-producing diets, which we have shown to decrease numerous inflammatory western diseases in mouse models. These diets operate at many levels - increased gut integrity, changes to the gut microbiome, and promotion of immune homeostasis, which represents a new and highly promising way to prevent or treat human disease.

The Role of a Novel Generation of Emollients, 'Emollients Plus', in Atopic Dermatitis

Emollients are the mainstay maintenance treatment for atopic dermatitis (AD). A novel generation of emollients, 'emollients plus', containing active, non-medicated substances, has softened the distinction between emollients and topical drugs. A literature search for selected key words was performed using PubMed. Additional papers were identified based on author expertise. Whilst the inclusion of five components of an ideal emollient has been proposed, no such consensus exists for emollients plus and they can vary markedly in their composition and modes of action for AD treatment. This could have a profound effect on their clinical efficacy. The efficacy of emollients plus in restoring and maintaining skin barrier function has been demonstrated on multiple levels, with evidence reported for their effects on the physical and biochemical, microbial, immunological, and neurosensory barriers. When selecting an appropriate AD treatment approach, the safety profiles of the available topical therapies must be carefully considered. There are several proposed treatment approaches for AD, including preventive, proactive, intermittent, and synergistic approaches. Emollients plus may be effective not only as maintenance therapy for AD, but also when used synergistically with anti-inflammatory pharmacological therapies.

Germinal Center-Related G Protein-Coupled Receptors in Antibody-Mediated Autoimmune Skin Diseases: from Basic Research to Clinical Trials

Germinal center (GC) reaction greatly contributes to the humoral immune response, which begins in lymph nodes or other secondary lymphoid organs after follicular B cells are activated by T-dependent antigens. The GCs then serve as a platform for follicular B cells to complete clonal expansion and somatic hypermutation and then interact with follicular dendritic cells (FDC) and follicular helper T cells (Tfh). Through the interaction between the immune cells, significant processes of the humoral immune response are accomplished, such as antibody affinity maturation, class switching, and production of memory B cells and plasma cells. Cell positioning during the GC reaction is mainly mediated by the chemokine receptors and lipid receptors, which both belong to G protein-coupled receptors (GPCRs) family. There are some orphan GPCRs whose endogenous ligands are unclear yet contribute to the regulation of GC reaction as well. This review will give an introduction on the ligands and functions of two types of GC-relating GPCRs-chemokine receptors like CXCR4 and CXCR5, as well as emerging de-orphanized GPCRs like GPR183, GPR174, and P2RY8. The roles these GPCRs play in several antibody-mediated autoimmune skin diseases will be also discussed, including systemic lupus erythematosus (SLE), pemphigus, scleroderma, and dermatomyositis. Besides, GPCRs are excellent drug targets due to the unique structure and vital functions. Therefore, this review is aimed at providing readers with a focused knowledge about the role that GPCRs play in GC reaction, as well as in provoking the development of GPCR-targeting agents for immune-mediated diseases besides autoimmune diseases.

Physiological relevance of proton-activated GPCRs

The detection of H⁺ concentration variations in the extracellular milieu is accomplished by a series of specialized and non-specialized pH-sensing mechanisms. The proton-activated G protein–coupled receptors (GPCRs) GPR4 (Gpr4), TDAG8 (Gpr65), and OGR1 (Gpr68) form a subfamily of proteins capable of triggering intracellular signaling in response to alterations in extracellular pH around physiological values, i.e., in the range between pH 7.5 and 6.5. Expression of these receptors is widespread for GPR4 and OGR1 with particularly high levels in endothelial cells and vascular smooth muscle cells, respectively, while expression of TDAG8 appears to be more restricted to the immune compartment. These receptors have been linked to several well-studied pH-dependent physiological activities including central control of respiration, renal adaption to changes in acid–base status, secretion of insulin and peripheral responsiveness to insulin, mechanosensation, and cellular chemotaxis. Their role in pathological processes such as the genesis and progression of several inflammatory diseases (asthma, inflammatory bowel disease), and tumor cell metabolism and invasiveness, is increasingly receiving more attention and makes these receptors novel and interesting targets for therapy. In this review, we cover the role of these receptors in physiological processes and will briefly discuss some implications for disease processes.

pH and Proton Sensor GPR65 Determine Susceptibility to Atopic Dermatitis

pH sensing by GPR65 regulates various inflammatory conditions, but its role in skin remains unknown. In this study, we performed a phenome-wide association study and report that the T allele of GPR65-intronic single-nucleotide polymorphism rs8005161, which reduces GPR65 signaling, showed a significant association with atopic dermatitis, in addition to inflammatory bowel diseases and asthma, as previously reported. Consistent with this genetic association in humans, we show that deficiency of GPR65 in mice resulted in markedly exacerbated disease in the MC903 experimental model of atopic dermatitis. Deficiency of GPR65 also increased neutrophil migration in vitro. Moreover, GPR65 deficiency in mice resulted in higher expression of the inflammatory cytokine TNF-α by T cells. In humans, CD4⁺ T cells from rs8005161 heterozygous individuals expressed higher levels of TNF-α after PMA/ionomycin stimulation, particularly under pH 6 conditions. pH sensing by GPR65 appears to be important for regulating the pathogenesis of atopic dermatitis.

A review of vaccine effects on women in light of the COVID-19 pandemic

The pandemic situation triggered by the spread of COVID-19 has caused great harm worldwide. More than six million people have been infected, and more than 360,000 of them have died. This is the worst catastrophe suffered by mankind in recent history. In the face of this severe disaster, people all over the world are frightened of the prospect of facing an outbreak or an annual recurrence. However, the development of a vaccine will help control the impact of COVID-19. Women in particular have been more seriously affected by the pandemic. Since the pressure and physical load they suffer are often greater than what men endure, women are more threatened by COVID-19. Though women have a poorer quality of life and work and face worse economic conditions, they also tend to have better physiological immunity than men, which can ease the effect of COVID-19. The early development of a vaccine against COVID-19 is an important issue that must take into consideration women's better immune response to the virus along with the technique of hormone regulation. Relevant research has been conducted on female-specific vaccines in the past, and women's issues were considered during those clinical trials to ensure that complications and antibody responses were positive and effective in women. National policies should also propose good strategies for women to be vaccinated. This could improve consciousness, give women a better vaccination experience, enhance their willingness to vaccinate, and protect them from COVID-19 infection.

The Lysophosphatidylserines-An Emerging Class of Signalling Lysophospholipids

Lysophospholipids are potent hormone-like signalling biological lipids that regulate many important biological processes in mammals (including humans). Lysophosphatidic acid and sphingosine-1-phosphate represent the best studied examples for this lipid class, and their metabolic enzymes and/or cognate receptors are currently under clinical investigation for treatment of various neurological and autoimmune diseases in humans. Over the past two decades, the lysophsophatidylserines (lyso-PSs) have emerged as yet another biologically important lysophospholipid, and deregulation in its metabolism has been linked to various human pathophysiological conditions. Despite its recent emergence, an exhaustive review summarizing recent advances on lyso-PSs and the biological pathways that this bioactive lysophospholipid regulates has been lacking. To address this, here, we summarize studies that led to the discovery of lyso-PS as a potent signalling biomolecule, and discuss the structure, its detection in biological systems, and the biodistribution of this lysophospholipid in various mammalian systems. Further, we describe in detail the enzymatic pathways that are involved in the biosynthesis and degradation of this lipid and the putative lyso-PS receptors reported in the literature. Finally, we discuss the various biological pathways directly regulated by lyso-PSs in mammals and prospect new questions for this still emerging biomedically important signalling lysophospholipid.

Proton-sensing G protein-coupled receptors: detectors of tumor acidosis and candidate drug targets

Cells in tumor microenvironments (TMEs) use several mechanisms to sense their low pH (<7.0), including via proton-sensing G protein-coupled receptors (psGPCRs): GPR4, GPR65/TDAG8, GPR68/OGR1 and GPR132/G2A. Numerous cancers have increased expression of psGPCRs. The psGPCRs may contribute to features of the malignant phenotype via actions on specific cell-types in the TME and thereby promote tumor survival and growth. Here, we review data regarding psGPCR expression in tumors and cancer cells, impact of psGPCRs on survival in solid tumors and a bioinformatics approach to infer psGPCR expression in cell types in the TME. New tools are needed to help define contributions of psGPCRs in tumor biology and to identify potentially novel therapeutic agents for a variety of cancers.

The increased marginal zone B cells attenuates early inflammatory responses during sepsis in Gpr174 deficient mice

GPR174 plays a crucial role in immune responses, but the role of GPR174 in the pathological progress of sepsis remains incompletely understood. In this study, we generated a sepsis model by cecal ligation and puncture (CLP) to investigate the role of GPR174 in regulating functions and underlying mechanism of marginal zone B (MZ B) cells in sepsis. We found that in Gpr174 deficient mice, the number of splenic MZ B cells was increased. Moreover, Gpr174^-/- MZ B cells exhibited an enhanced response to LPS stimulation in vitro. By using the CLP-induced sepsis model, we demonstrated that the increased MZ B cells attenuated early inflammatory responses during sepsis. RNA sequencing results revealed that the expression of c-fos in splenic B lymphocytes was upregulated in Gpr174 deficient mice. However, the protective role of increased MZ B cells in Gpr174 deficient mice was weakened by a c-fos-specific inhibitor. Collectively, these findings suggested that GPR174 plays an immunomodulatory role in early immune responses during sepsis through the regulation of MZ B cells.