Glycoursodeoxycholic Acid Alleviates Arterial Thrombosis via Suppressing Diacylglycerol Kinases Activity in Platelet

BACKGROUND

Glycoursodeoxycholic acid (GUDCA) has been acknowledged for its ability to regulate lipid homeostasis and provide benefits for various metabolic disorders. However, the impact of GUDCA on arterial thrombotic events remains unexplored. The objective of this study is to examine the effects of GUDCA on thrombogenesis and elucidate its underlying mechanisms.

METHODS

Plasma samples from patients with arterial thrombotic events and diet-induced obese mice were collected to determine the GUDCA concentrations using mass spectrometry. Multiple in vivo murine thrombosis models and in vitro platelet functional assays were conducted to comprehensively evaluate the antithrombotic effects of GUDCA. Moreover, lipidomic analysis was performed to identify the alterations of intraplatelet lipid components following GUDCA treatment.

RESULTS

Plasma GUDCA level was significantly decreased in patients with arterial thrombotic events and negatively correlated with thrombotic propensity in diet-induced obese mice. GUDCA exhibited prominent suppressing effects on platelet reactivity as evidenced by the attenuation of platelet activation, secretion, aggregation, spreading, and retraction (P<0.05). In vivo, GUDCA administration robustly alleviated thrombogenesis (P<0.05) without affecting hemostasis. Mechanistically, GUDCA inhibited DGK (diacylglycerol kinase) activity, leading to the downregulation of the phosphatidic acid-mediated signaling pathway. Conversely, phosphatidic acid supplementation was sufficient to abolish the antithrombotic effects of GUDCA. More importantly, long-term oral administration of GUDCA normalized the enhanced DGK activity, thereby remarkably alleviating the platelet hyperreactivity as well as the heightened thrombotic tendency in diet-induced obese mice (P<0.05).

CONCLUSIONS

Our study implicated that GUDCA reduces platelet hyperreactivity and improves thrombotic propensity by inhibiting DGKs activity, which is a potentially effective prophylactic approach and promising therapeutic agent for arterial thrombotic events.

Lipid switches in the immunological synapse

Adaptive immune responses comprise the activation of T cells by peptide antigens that are presented by proteins of the Major Histocompatibility Complex (MHC) on the surface of an antigen-presenting cell (APC). As a consequence of the T cell receptor (TCR) interacting productively with a certain peptide-MHC complex, a specialized cell-cell junction, the immunological synapse, forms and is accompanied by changes in the spatiotemporal patterning and function of intracellular signaling molecules. Key modifications occurring at the cytoplasmic leaflet of the plasma and internal membranes in activated T cells comprise lipid switches that affect the binding and distribution of proteins within or near the lipid bilayer. Here, we describe two major classes of lipid switches that act at this critical water/membrane interface. Phosphoinositides are derived from phosphatidylinositol, an amphiphilic molecule that contains two fatty acids chains and a phosphate group that bridges the glycerol backbone to the carbohydrate inositol. The inositol ring can be variably (de-)phosphorylated by dedicated kinases and phosphatates, thereby creating phosphoinositide signatures that define the composition and properties of signaling molecules, molecular complexes or whole organelles. Palmitoylation refers to the reversible attachment of the fatty acid palmitate to a substrate protein's cysteine residue. DHHC enzymes, named after the four conserved amino acids in their active site, catalyze this post-translational modification and thereby change the distribution of proteins at, between and within membranes. T cells utilize these two types of molecular switches to adjust their properties to an activation process that requires changes in motility, transport, secretion and gene expression.

Diacylglycerol kinases: A look into the future of immunotherapy

Cancer still represents the second leading cause of death right after cardiovascular diseases. According to the World Health Organization (WHO), cancer provoked around 10 million deaths in 2020, with lung and colon tumors accounting for the deadliest forms of cancer. As tumor cells become resistant to traditional therapeutic approaches, immunotherapy has emerged as a novel strategy for tumor control. T lymphocytes are key players in immune responses against tumors. Immunosurveillance allows identification, targeting and later killing of cancerous cells. Nevertheless, tumors evolve through different strategies to evade the immune response and spread in a process called metastasis. The ineffectiveness of traditional strategies to control tumor growth and expansion has led to novel approaches considering modulation of T cell activation and effector functions. Program death receptor 1 (PD-1) and cytotoxic T-lymphocyte antigen 4 (CTLA-4) showed promising results in the early 90s and nowadays are still being exploited together with other drugs for several cancer types. Other negative regulators of T cell activation are diacylglycerol kinases (DGKs) a family of enzymes that catalyze the conversion of diacylglycerol (DAG) into phosphatidic acid (PA). In T cells, DGKα and DGKζ limit the PLCγ/Ras/ERK axis thus attenuating DAG mediated signaling and T cell effector functions. Upregulation of either of both isoforms results in impaired Ras activation and anergy induction, whereas germline knockdown mice showed enhanced antitumor properties and more effective immune responses against pathogens. Here we review the mechanisms used by DGKs to ameliorate T cell activation and how inhibition could be used to reinvigorate T cell functions in cancer context. A better knowledge of the molecular mechanisms involved upon T cell activation will help to improve current therapies with DAG promoting agents.

Overarching roles of diacylglycerol signaling in cancer development and antitumor immunity

Diacylglycerol (DAG) is a lipid second messenger that is generated in response to extracellular stimuli and channels intracellular signals that affect mammalian cell proliferation, survival, and motility. DAG exerts a myriad of biological functions through protein kinase C (PKC) and other effectors, such as protein kinase D (PKD) isozymes and small GTPase-regulating proteins (such as RasGRPs). Imbalances in the fine-tuned homeostasis between DAG generation by phospholipase C (PLC) enzymes and termination by DAG kinases (DGKs), as well as dysregulation in the activity or abundance of DAG effectors, have been widely associated with tumor initiation, progression, and metastasis. DAG is also a key orchestrator of T cell function and thus plays a major role in tumor immunosurveillance. In addition, DAG pathways shape the tumor ecosystem by arbitrating the complex, dynamic interaction between cancer cells and the immune landscape, hence representing powerful modifiers of immune checkpoint and adoptive T cell-directed immunotherapy. Exploiting the wide spectrum of DAG signals from an integrated perspective could underscore meaningful advances in targeted cancer therapy.

Sorting Nexin 27 Enables MTOC and Secretory Machinery Translocation to the Immune Synapse

Sorting nexin 27 (SNX27) association to the retromer complex mediates intracellular trafficking of cargoes containing PSD95/Dlg1/ZO-1 (PDZ)-binding C-terminal sequences from endosomes to the cell surface, preventing their lysosomal degradation. Antigen recognition by T lymphocyte leads to the formation of a highly organized structure named the immune synapse (IS), which ensures cell-cell communication and sustained T cell activation. At the neuronal synapse, SNX27 recycles PDZ-binding receptors and its defective expression is associated with synaptic dysfunction and cognitive impairment. In T lymphocytes, SNX27 was found localized at recycling endosomal compartments that polarized to the IS, suggesting a function in polarized traffic to this structure. Proteomic analysis of PDZ-SNX27 interactors during IS formation identify proteins with known functions in cytoskeletal reorganization and lipid regulation, such as diacylglycerol (DAG) kinase (DGK) ζ, as well as components of the retromer and WASH complex. In this study, we investigated the consequences of SNX27 deficiency in cytoskeletal reorganization during IS formation. Our analyses demonstrate that SNX27 controls the polarization towards the cell-cell interface of the PDZ-interacting cargoes DGKζ and the retromer subunit vacuolar protein sorting protein 26, among others. SNX27 silencing abolishes the formation of a DAG gradient at the IS and prevents re-localization of the dynactin complex component dynactin-1/p150^Glued, two events that correlate with impaired microtubule organizing center translocation (MTOC). SNX27 silenced cells show marked alteration in cytoskeleton organization including a failure in the organization of the microtubule network and defects in actin clearance at the IS. Reduced SNX27 expression was also found to hinder the arrangement of signaling microclusters at the IS, as well as the polarization of the secretory machinery towards the antigen presenting cells. Our results broaden the knowledge of SNX27 function in T lymphocytes by showing a function in modulating IS organization through regulated trafficking of cargoes.

Staggered starts in the race to T cell activation

How T lymphocytes tune their responses to different strengths of stimulation is a fundamental question in immunology. Recent work using new optogenetic, single-cell genomic, and live-imaging approaches has revealed that stimulation strength controls the rate of individual cell responses within a population. Moreover, these responses have been found to use shared molecular programs, regardless of stimulation strength. However, additional data indicate that stimulation duration or cytokine feedback can impact later gene expression phenotypes of activated cells. In-depth molecular studies have suggested mechanisms by which stimulation strength might modulate the probability of T cell activation. This emerging model allows activating T cells to achieve a wide range of population responses through probabilistic control within individual cells.

Diacylglycerol kinase ζ limits IL-2-dependent control of PD-1 expression in tumor-infiltrating T lymphocytes

Background

The inhibitory functions triggered by the programmed cell death-1 (PD-1) receptor following binding to its ligand (PD-L1) protect healthy organs from cytotoxic T cells, and neutralize antitumor T cell attack. Antibody-based therapies to block PD-1/PD-L1 interaction have yielded notable results, but most patients eventually develop resistance. This failure is attributed to CD8⁺ T cells achieving hyporesponsive states from which recovery is hardly feasible. Dysfunctional T cell phenotypes are favored by a sustained imbalance in the diacylglycerol (DAG)- and Ca²⁺-regulated transcriptional programs. In mice, DAG kinase ζ (DGKζ) facilitates DAG consumption, limiting T cell activation and cytotoxic T cell responses. DGKζ deficiency facilitates tumor rejection in mice without apparent adverse autoimmune effects. Despite its therapeutic potential, little is known about DGKζ function in human T cells, and no known inhibitors target this isoform.

Methods

We used a human triple parameter reporter cell line to examine the consequences of DGKζ depletion on the transcriptional restriction imposed by PD-1 ligation. We studied the effect of DGKζ deficiency on PD-1 expression dynamics, as well as the impact of DGKζ absence on the in vivo growth of MC38 adenocarcinoma cells.

Results

We demonstrate that DGKζ depletion enhances DAG-regulated transcriptional programs, promoting interleukin-2 production and partially counteracting PD-1 inhibitory functions. DGKζ loss results in limited PD-1 expression and enhanced expansion of cytotoxic CD8⁺ T cell populations. This is observed even in immunosuppressive milieus, and correlates with the reduced ability of MC38 adenocarcinoma cells to form tumors in DGKζ-deficient mice.

Conclusions

Our results, which define a role for DGKζ in the control of PD-1 expression, confirm DGKζ potential as a therapeutic target as well as a biomarker of CD8⁺ T cell dysfunctional states.

Potential role of diacylglycerol kinases in immune-mediated diseases

The mechanism promoting exacerbated immune responses in allergy and autoimmunity as well as those blunting the immune control of cancer cells are of primary interest in medicine. Diacylglycerol kinases (DGKs) are key modulators of signal transduction, which blunt diacylglycerol (DAG) signals and produce phosphatidic acid (PA). By modulating lipid second messengers, DGK modulate the activity of downstream signaling proteins, vesicle trafficking and membrane shape. The biological role of the DGK α and ζ isoforms in immune cells differentiation and effector function was subjected to in deep investigations. DGK α and ζ resulted in negatively regulating synergistic way basal and receptor induced DAG signals in T cells as well as leukocytes. In this way, they contributed to keep under control the immune response but also downmodulate immune response against tumors. Alteration in DGKα activity is also implicated in the pathogenesis of genetic perturbations of the immune function such as the X-linked lymphoproliferative disease 1 and localized juvenile periodontitis. These findings suggested a participation of DGK to the pathogenetic mechanisms underlying several immune-mediated diseases and prompted several researches aiming to target DGK with pharmacologic and molecular strategies. Those findings are discussed inhere together with experimental applications in tumors as well as in other immune-mediated diseases such as asthma.

Interplay Between SNX27 and DAG Metabolism in the Control of Trafficking and Signaling at the IS

Recognition of antigens displayed on the surface of an antigen-presenting cell (APC) by T-cell receptors (TCR) of a T lymphocyte leads to the formation of a specialized contact between both cells named the immune synapse (IS). This highly organized structure ensures cell–cell communication and sustained T-cell activation. An essential lipid regulating T-cell activation is diacylglycerol (DAG), which accumulates at the cell–cell interface and mediates recruitment and activation of proteins involved in signaling and polarization. Formation of the IS requires rearrangement of the cytoskeleton, translocation of the microtubule-organizing center (MTOC) and vesicular compartments, and reorganization of signaling and adhesion molecules within the cell–cell junction. Among the multiple players involved in this polarized intracellular trafficking, we find sorting nexin 27 (SNX27). This protein translocates to the T cell–APC interface upon TCR activation, and it is suggested to facilitate the transport of cargoes toward this structure. Furthermore, its interaction with diacylglycerol kinase ζ (DGKζ), a negative regulator of DAG, sustains the precise modulation of this lipid and, thus, facilitates IS organization and signaling. Here, we review the role of SNX27, DAG metabolism, and their interplay in the control of T-cell activation and establishment of the IS.

Diacylglycerol kinase ζ promotes actin cytoskeleton remodeling and mechanical forces at the B cell immune synapse

Diacylglycerol kinases (DGKs) limit antigen receptor signaling in immune cells by consuming the second messenger diacylglycerol (DAG) to generate phosphatidic acid (PA). Here, we showed that DGKζ promotes lymphocyte function-associated antigen 1 (LFA-1)-mediated adhesion and F-actin generation at the immune synapse of B cells with antigen-presenting cells (APCs), mostly in a PA-dependent manner. Measurement of single-cell mechanical force generation indicated that DGKζ-deficient B cells exerted lower forces at the immune synapse than did wild-type B cells. Nonmuscle myosin activation and translocation of the microtubule-organizing center (MTOC) to the immune synapse were also impaired in DGKζ-deficient B cells. These functional defects correlated with the decreased ability of B cells to present antigen and activate T cells in vitro. The in vivo germinal center response of DGKζ-deficient B cells was also reduced compared with that of wild-type B cells, indicating that loss of DGKζ in B cells impaired T cell help. Together, our data suggest that DGKζ shapes B cell responses by regulating actin remodeling, force generation, and antigen uptake-related events at the immune synapse. Hence, an appropriate balance in the amounts of DAG and PA is required for optimal B cell function.

A PI(4,5)P2-derived "gasoline engine model" for the sustained B cell receptor activation

To efficiently initiate activation responses against rare ligands in the microenvironment, lymphocytes employ sophisticated mechanisms involving signaling amplification. Recently, a signaling amplification mechanism initiated from phosphatidylinositol (PI) 4, 5-biphosphate [PI(4,5)P2] hydrolysis and synthesis for sustained B cell activation has been reported. Antigen and B cell receptor (BCR) recognition triggered the prompt reduction of PI(4,5)P2 density within the BCR microclusters, which led to the positive feedback for the synthesis of PI(4,5)P2 outside of the BCR microclusters. At single molecule level, the diffusion of PI(4,5)P2 was slow, allowing for the maintenance of a PI(4,5)P2 density gradient between the inside and outside of the BCR microclusters and the persistent supply of PI(4,5)P2 from outside to inside of the BCR microclusters. Here, we review studies that have contributed to uncovering the molecular mechanisms of PI(4,5)P2-derived signaling amplification model. Based on these studies, we proposed a "gasoline engine model" in which the activation of B cell signaling inside the microclusters is similar to the working principle of burning gasoline within the engine chamber of a gasoline engine. We also discuss the evidences showing the potential universality of this model and future prospects.

Diacylglycerol kinase control of protein kinase C

The diacylglycerol kinases (DGK) are lipid kinases that transform diacylglycerol (DAG) into phosphatidic acid (PA) in a reaction that terminates DAG-based signals. DGK provide negative regulation to conventional and novel protein kinase C (PKC) enzymes, limiting local DAG availability in a tissue- and subcellular-restricted manner. Defects in the expression/activity of certain DGK isoforms contribute substantially to cognitive impairment and mental disorders. Abnormal DGK overexpression in tumors facilitates invasion and resistance to chemotherapy preventing tumor immune destruction by tumor-infiltrating lymphocytes. Effective translation of these findings into therapeutic approaches demands a better knowledge of the physical and functional interactions between the DGK and PKC families. DGKζ is abundantly expressed in the nervous and immune system, where physically and functionally interacts with PKCα. The latest discoveries suggest that PDZ-mediated interaction facilitates spatial restriction of PKCα by DGKζ at the cell–cell contact sites in a mechanism where the two enzymes regulate each other. In T lymphocytes, DGKζ interaction with Sorting Nexin 27 (SNX27) guarantees the basal control of PKCα activation. SNX27 is a trafficking component required for normal brain function whose deficit has been linked to Alzheimer's disease (AD) pathogenesis. The enhanced PKCα activation as the result of SNX27 silencing in T lymphocytes aligns with the recent correlation found between gain-of-function PKCα mutations and AD and suggests that disruption of the mechanisms that provides a correct spatial organization of DGKζ and PKCα may lie at the basis of immune and neuronal synapse impairment.

SNX27 links DGKζ to the control of transcriptional and metabolic programs in T lymphocytes

Sorting nexin 27 (SNX27) recycles PSD-95, Dlg1, ZO-1 (PDZ) domain-interacting membrane proteins and is essential to sustain adequate brain functions. Here we define a fundamental SNX27 function in T lymphocytes controlling antigen-induced transcriptional activation and metabolic reprogramming. SNX27 limits the activation of diacylglycerol (DAG)-based signals through its high affinity PDZ-interacting cargo DAG kinase ζ (DGKζ). SNX27 silencing in human T cells enhanced T cell receptor (TCR)-stimulated activator protein 1 (AP-1)- and nuclear factor κB (NF-κB)-mediated transcription. Transcription did not increase upon DGKζ silencing, suggesting that DGKζ function is dependent on SNX27. The enhanced transcriptional activation in SNX27-silenced cells contrasted with defective activation of the mammalian target of rapamycin (mTOR) pathway. The analysis of Snx27^−/− mice supported a role for SNX27 in the control of T cell growth. This study broadens our understanding of SNX27 as an integrator of lipid-based signals with the control of transcription and metabolic pathways.

Aplastic anemia is related to alterations in T cell receptor signaling

Aplastic anemia (AA) is a disease characterized by bone marrow hematopoietic dysfunction and peripheral blood pancytopenia, which is thought to be mediated by an abnormal T cell-induced immune response. T cell receptor (TCR) signaling is pivotal for T cell development and function. An aberrant TCR signaling leads to an unbalanced immune system that can result in a range of immune-related disorders, including autoimmune diseases, chronic infections, and tumors. In this article, we briefly review the T cell immune pathophysiology of AA, the physiology of normal TCR signaling and its regulatory factors, and clinical and laboratory findings of TCR signaling molecules and their regulatory factors in AA.