TAM receptors and the clearance of apoptotic cells

Dexmedetomidine attenuates sepsis-associated acute lung injury by regulating macrophage efferocytosis through the ROS/ADAM10/AXL pathway

BACKGROUND

The lungs are highly susceptible to damage during sepsis, with severe lung injury potentially progressing to acute respiratory distress syndrome and even fatal sepsis. Effective efferocytosis of apoptotic cells is crucial in alleviating inflammation and tissue injury.

METHODS

We established a septic lung injury mouse model via intraperitoneal injection of lipopolysaccharide. Lung injury was assessed by histology, immunofluorescence, neutrophil immunohistochemistry staining, and cytokine detection. We extracted alveolar macrophages by bronchoalveolar lavage and primary macrophages from mouse bone marrow to investigate the regulatory effects of Dexmedetomidine (DEX) on efferocytosis. We further validated the molecular mechanisms underlying the regulation of macrophage efferocytosis by DEX through knockdown of AXL expression. Additionally, we examined the efferocytic ability of monocytes isolated from patients.

RESULTS

We discovered that DEX treatment effectively alleviated pulmonary injury and inflammation. Lipopolysaccharide reduced macrophage efferocytosis and AXL expression which were reversed by DEX. We also found DEX inhibited the increased activation of A Disintegrin And Metalloproteinase 10 (ADAM10) and the production of soluble AXL. Moreover, our findings demonstrated that DEX decreased the elevated ROS production linked to higher ADAM10 activation. Blocking AXL negated DEX's benefits on efferocytosis and lung protection. Efferocytosis in monocytes from septic lung injury patients was notably lower than in healthy individuals.

CONCLUSION

Our findings demonstrated that DEX treatment effectively reduces septic lung injury by promoting macrophage efferocytosis through ROS/ADAM10/AXL signaling pathwway.

Microparticles incorporating dual apoptotic factors to inhibit inflammatory effects in macrophages

New approaches to treat autoimmune diseases are needed, and we can be inspired by mechanisms in immune tolerance to guide the design of these approaches. Efferocytosis, the process of phagocyte-mediated apoptotic cell (AC) disposal, represents a potent tolerogenic mechanism that we could draw inspiration from to restore immune tolerance to specific autoantigens. ACs engage multiple avenues of the immune response to redirect aberrant immune responses. Two such avenues are: phosphatidylserine on the outer leaflet of the cell and engaging the aryl hydrocarbon receptor (AhR) pathway. We incorporated these two avenues into one acetalated dextran (Ace-DEX) microparticle (MP) for evaluation in vitro. First phosphatidylserine (PS) was incorporated into Ace-DEX MPs and evaluated for cellular association and mediators of cell tolerance including IL-10 production and M2 associated gene expression when particles were cultured with peritoneal macrophages (PMacs). Further PS Ace-DEX MPs were evaluated as an agent to suppress LPS stimulated PMacs. Then, AhR agonist 2-(1'H-indole-3'-carbonyl)-thiazole-4-carboxylic acid methyl ester (ITE) was incorporated into Ace-DEX MPs and expression of M2 and IL-10 genes was evaluated in PMacs. Further the ITE and PS Ace-DEX MPs (PS/ITE MPs) were evaluated for suppression of T cell priming and Th1 polarization. Our results indicate that the PS/ITE-MPs stimulated anti-inflammatory cytokine expression and suppressed inflammation following LPS stimulation of PMacs. Moreover, PS/ITE MPs induced the anti-inflammatory enzyme IDO1 and suppressed macrophage-mediated T cell priming and Th1 polarization. These findings suggest that PS and ITE-loaded Ace-DEX MPs could be a promising therapeutic tool for suppressing inflammation.

GAS6 and AXL Promote Insulin Resistance by Rewiring Insulin Signaling and Increasing Insulin Receptor Trafficking to Endosomes

Growth arrest-specific 6 (GAS6) is a secreted protein that acts as a ligand for TAM receptors (TYRO3, AXL, and MERTK). In humans, GAS6 circulating levels and genetic variations in GAS6 are associated with hyperglycemia and increased risk of type 2 diabetes. However, the mechanisms by which GAS6 influences glucose metabolism are not understood. Here, we show that Gas6 deficiency in mice increases insulin sensitivity and protects from diet-induced insulin resistance. Conversely, increasing GAS6 circulating levels is sufficient to reduce insulin sensitivity in vivo. GAS6 inhibits the activation of the insulin receptor (IR) and reduces insulin response in muscle cells in vitro and in vivo. Mechanistically, AXL and IR form a complex, while GAS6 reprograms signaling pathways downstream of IR. This results in increased IR endocytosis following insulin treatment. This study contributes to a better understanding of the cellular and molecular mechanisms by which GAS6 and AXL influence insulin sensitivity.

ARTICLE HIGHLIGHTS

Housing RCS rats under specific pathogen-free conditions mildly ameliorates retinal degeneration and alters intestine microbiota

Retinitis pigmentosa (RP) is a genetic blinding disease with over 80 causative genes. Disease progression varies between patients with similar genetic backgrounds. We assessed the association between environment, gut microbiota, and retinal degeneration in the RP rat model Royal College of Surgeons (RCS). The rats were born and raised for two generations under specific pathogen-free (SPF, n = 69) or non-SPF conditions (n = 48). At the age of four weeks, SPF rats had significantly shorter dark-adapted a-wave and dark and light-adapted b-wave implicit times by electroretinogram (p = 0.014, p = 9.5*10-6, p = 0.009, respectively). The SPF rats had significantly less photoreceptor apoptosis at ages four, eight, and twelve weeks (all p < 0.022), significantly thicker debris zone at age 14 weeks, and smaller hypofluorescent lesions in SPF rats at ages 10-16 weeks, especially in the inferior retina. The non-SPF rats had significantly higher microbiota alpha diversity (p = 0.037) and failed to present the age-related maturation of Proteobacteria, Spirochaetes, Actinobacteria, and Bacteroidetes seen in SPF conditions. Specific microbial amplicon sequence variants were reduced in rats with more severe retinal degeneration. Our data suggest an environmental effect on retinal deterioration in RCS rats. These findings may lead to the development of novel microbiome-related interventions for retinal degeneration.

Therapeutic biomaterials with liver X receptor agonists based on the horizon of material biology to regulate atherosclerotic plaque regression in situ for devices surface engineering

Percutaneous coronary interventional is the main treatment for coronary atherosclerosis. At present, most studies focus on blood components and smooth muscle cells to achieve anticoagulation or anti-proliferation effects, while the mediated effects of materials on macrophages are also the focus of attention. Macrophage foam cells loaded with elevated cholesterol is a prominent feature of atherosclerotic plaque. Activation of liver X receptor (LXR) to regulate cholesterol efflux and efferocytosis and reduce the number of macrophage foam cells in plaque is feasible for the regression of atherosclerosis. However, cholesterol efflux promotion remains confined to targeted therapies. Herein, LXR agonists (GW3965) were introduced on the surface of the material and delivered in situ to atherogenic macrophages to improve drug utilization for anti-atherogenic therapy and plaque regression. LXR agonists act as plaque inhibition mediated by multichannel regulation macrophages, including lipid metabolism (ABCA1, ABCG1 and low-density lipoprotein receptor), macrophage migration (CCR7) and efferocytosis (MerTK). Material loaded with LXR agonists significantly reduced plaque burden in atherosclerotic model rats, most importantly, it did not cause hepatotoxicity and adverse reactions such as restenosis and thrombosis after material implantation. Both in vivo and in vitro evaluations confirmed its anti-atherosclerotic capability and safety. Overall, multi-functional LXR agonist-loaded materials with pathological microenvironment regulation effect are expected to be promising candidates for anti-atherosclerosis and have potential applications in cardiovascular devices surface engineering.

Targeting Macrophage Polarization for Reinstating Homeostasis following Tissue Damage

Tissue regeneration and remodeling involve many complex stages. Macrophages are critical in maintaining micro-environmental homeostasis by regulating inflammation and orchestrating wound healing. They display high plasticity in response to various stimuli, showing a spectrum of functional phenotypes that vary from M1 (pro-inflammatory) to M2 (anti-inflammatory) macrophages. While transient inflammation is an essential trigger for tissue healing following an injury, sustained inflammation (e.g., in foreign body response to implants, diabetes or inflammatory diseases) can hinder tissue healing and cause tissue damage. Modulating macrophage polarization has emerged as an effective strategy for enhancing immune-mediated tissue regeneration and promoting better integration of implantable materials in the host. This article provides an overview of macrophages' functional properties followed by discussing different strategies for modulating macrophage polarization. Advances in the use of synthetic and natural biomaterials to fabricate immune-modulatory materials are highlighted. This reveals that the development and clinical application of more effective immunomodulatory systems targeting macrophage polarization under pathological conditions will be driven by a detailed understanding of the factors that regulate macrophage polarization and biological function in order to optimize existing methods and generate novel strategies to control cell phenotype.

Overnutrition and Lipotoxicity: Impaired Efferocytosis and Chronic Inflammation as Precursors to Multifaceted Disease Pathogenesis

Overnutrition, driven by the consumption of high-fat, high-sugar diets, has reached epidemic proportions and poses a significant global health challenge. Prolonged overnutrition leads to the deposition of excessive lipids in adipose and non-adipose tissues, a condition known as lipotoxicity. The intricate interplay between overnutrition-induced lipotoxicity and the immune system plays a pivotal role in the pathogenesis of various diseases. This review aims to elucidate the consequences of impaired efferocytosis, caused by lipotoxicity-poisoned macrophages, leading to chronic inflammation and the subsequent development of severe infectious diseases, autoimmunity, and cancer, as well as chronic pulmonary and cardiovascular diseases. Chronic overnutrition promotes adipose tissue expansion which induces cellular stress and inflammatory responses, contributing to insulin resistance, dyslipidemia, and metabolic syndrome. Moreover, sustained exposure to lipotoxicity impairs the efferocytic capacity of macrophages, compromising their ability to efficiently engulf and remove dead cells. The unresolved chronic inflammation perpetuates a pro-inflammatory microenvironment, exacerbating tissue damage and promoting the development of various diseases. The interaction between overnutrition, lipotoxicity, and impaired efferocytosis highlights a critical pathway through which chronic inflammation emerges, facilitating the development of severe infectious diseases, autoimmunity, cancer, and chronic pulmonary and cardiovascular diseases. Understanding these intricate connections sheds light on potential therapeutic avenues to mitigate the detrimental effects of overnutrition and lipotoxicity on immune function and tissue homeostasis, thereby paving the way for novel interventions aimed at reducing the burden of these multifaceted diseases on global health.

A single cell atlas of frozen shoulder capsule identifies features associated with inflammatory fibrosis resolution

Frozen shoulder is a spontaneously self-resolving chronic inflammatory fibrotic human disease, which distinguishes the condition from most fibrotic diseases that are progressive and irreversible. Using single-cell analysis, we identify pro-inflammatory MERTKlowCD48+ macrophages and MERTK + LYVE1 + MRC1+ macrophages enriched for negative regulators of inflammation which co-exist in frozen shoulder capsule tissues. Micro-cultures of patient-derived cells identify integrin-mediated cell-matrix interactions between MERTK+ macrophages and pro-resolving DKK3+ and POSTN+ fibroblasts, suggesting that matrix remodelling plays a role in frozen shoulder resolution. Cross-tissue analysis reveals a shared gene expression cassette between shoulder capsule MERTK+ macrophages and a respective population enriched in synovial tissues of rheumatoid arthritis patients in disease remission, supporting the concept that MERTK+ macrophages mediate resolution of inflammation and fibrosis. Single-cell transcriptomic profiling and spatial analysis of human foetal shoulder tissues identify MERTK + LYVE1 + MRC1+ macrophages and DKK3+ and POSTN+ fibroblast populations analogous to those in frozen shoulder, suggesting that the template to resolve fibrosis is established during shoulder development. Crosstalk between MerTK+ macrophages and pro-resolving DKK3+ and POSTN+ fibroblasts could facilitate resolution of frozen shoulder, providing a basis for potential therapeutic resolution of persistent fibrotic diseases.

GAS6/TAM Axis as Therapeutic Target in Liver Diseases

TAM (TYRO3, AXL, and MERTK) protein tyrosine kinase membrane receptors and their vitamin K-dependent ligands GAS6 and protein S (PROS) are well-known players in tumor biology and autoimmune diseases. In contrast, TAM regulation of fibrogenesis and the inflammation mechanisms underlying metabolic dysfunction-associated steatohepatitis (MASH), cirrhosis, and, ultimately, liver cancer has recently been revealed. GAS6 and PROS binding to phosphatidylserine exposed in outer membranes of apoptotic cells links TAMs, particularly MERTK, with hepatocellular damage. In addition, AXL and MERTK regulate the development of liver fibrosis and inflammation in chronic liver diseases. Acute hepatic injury is also mediated by the TAM system, as recent data regarding acetaminophen toxicity and acute-on-chronic liver failure have uncovered. Soluble TAM-related proteins, mainly released from activated macrophages and hepatic stellate cells after hepatic deterioration, are proposed as early serum markers for disease progression. In conclusion, the TAM system is becoming an interesting pharmacological target in liver pathology and a focus of future biomedical research in this field.

The role of efferocytosis and transplant rejection: Strategies in promoting transplantation tolerance using apoptotic cell therapy and/or synthetic particles

Recently, efforts have been made to recognize the precise reason(s) for transplant failure and the process of rejection utilizing the molecular signature. Most transplant recipients do not appreciate the unknown length of survival of allogeneic grafts with the existing standard of care. Two noteworthy immunological pathways occur during allogeneic transplant rejection. A nonspecific innate immune response predominates in the early stages of the immune reaction, and allogeneic antigens initiate a donor-specific adaptive reaction. Though the adaptive response is the major cause of allograft rejection, earlier pro-inflammatory responses that are part of the innate immune response are also regarded as significant in graft loss. The onset of the innate and adaptive immune response causes chronic and acute transplant rejection. Currently employed immunosuppressive medications have shown little or no influence on chronic rejection and, as a result, on overall long-term transplant survival. Furthermore, long-term pharmaceutical immunosuppression is associated with side effects, toxicity, and an increased risk of developing diseases, both infectious and metabolic. As a result, there is a need for the development of innovative donor-specific immunosuppressive medications to regulate the allorecognition pathways that induce graft loss and to reduce the side effects of immunosuppression. Efferocytosis is an immunomodulatory mechanism with fast and efficient clearance of apoptotic cells (ACs). As such, AC therapy strategies have been suggested to limit transplant-related sequelae. Efferocytosis-based medicines/treatments can also decrease the use of immunosuppressive drugs and have no detrimental side effects. Thus, this review aims to investigate the impact of efferocytosis on transplant rejection/tolerance and identify approaches using AC clearance to increase transplant viability.

TAM receptors in phagocytosis: Beyond the mere internalization of particles

TYRO3, AXL, and MERTK constitute the TAM family of receptor tyrosine kinases, activated by their ligands GAS6 and PROS1. TAMs are necessary for adult homeostasis in the immune, nervous, reproductive, skeletal, and vascular systems. Among additional cellular functions employed by TAMs, phagocytosis is central for tissue health. TAM receptors are dominant in providing phagocytes with the molecular machinery necessary to engulf diverse targets, including apoptotic cells, myelin debris, and portions of live cells in a phosphatidylserine-dependent manner. Simultaneously, TAMs drive the release of anti-inflammatory and tissue repair molecules. Disruption of the TAM-driven phagocytic pathway has detrimental consequences, resulting in autoimmunity, male infertility, blindness, and disrupted vascular integrity, and which is thought to contribute to neurodegenerative diseases. Although structurally and functionally redundant, the TAM receptors and ligands underlie complex signaling cascades, of which several key aspects are yet to be elucidated. We discuss similarities and differences between TAMs and other phagocytic pathways, highlight future directions and how TAMs can be harnessed therapeutically to modulate phagocytosis.

Inhibition of Mertk Signaling Enhances Bone Healing after Tooth Extraction

Regeneration of alveolar bone is an essential step in restoring healthy function following tooth extraction. Growth of new bone in the healing extraction socket can be variable and often unpredictable when systemic comorbidities are present, leading to the need for additional therapeutic targets to accelerate the regenerative process. One such target is the TAM family (Tyro3, Axl, Mertk) of receptor tyrosine kinases. These proteins have been shown to help resolve inflammation and maintain bone homeostasis and thus may have therapeutic benefits in bone regeneration following extraction. Treatment of mice with a pan-TAM inhibitor (RXDX-106) led to accelerated alveolar bone fill following first molar extraction in a mouse model without changing immune infiltrate. Treatment of human alveolar bone mesenchymal stem cells with RXDX-106 upregulated Wnt signaling and primed the cells for osteogenic differentiation. Differentiation of human alveolar bone mesenchymal stem cells with osteogenic media and TAM-targeted inhibitor RXDX-106 (pan-TAM), ASP-2215 (Axl specific), or MRX-2843 (Mertk specific) showed enhanced mineralization with pan-TAM or Mertk-specific inhibitors and no change with Axl-specific inhibitor. First molar extractions in Mertk-/- mice had increased alveolar bone regeneration in the extraction socket relative to wild type controls 7 d postextraction. Flow cytometry of 7-d extraction sockets showed no difference in immune cell numbers between Mertk-/- and wild type mice. RNAseq of day 7 extraction sockets showed increased innate immune-related pathways and genes associated with bone differentiation in Mertk-/- mice. Together, these results indicate that TAM receptor signaling, specifically through Mertk, can be targeted to enhance bone regeneration after injury.

After cell death: the molecular machinery of efferocytosis

Cells constituting a multicellular organism die in a variety of ways throughout life, and most of them die via apoptosis under normal conditions. The occurrence of apoptosis is especially prevalent during development and in tissues with a high cellular turnover rate, such as the thymus and bone marrow. Interestingly, although the number of apoptotic cells produced daily is known to be innumerable in a healthy adult human body, apoptotic cells are rarely observed. This absence is due to the existence of a cellular process called efferocytosis that efficiently clears apoptotic cells. Studies over the past decades have focused on how phagocytes are able to remove apoptotic cells specifically, swiftly, and continuously, resulting in defined molecular and cellular events. In this review, we will discuss the current understanding of the clearance of apoptotic cells at the molecular level.

Relationship between the tumor microenvironment and the efficacy of the combination of radiotherapy and immunotherapy

Activating and recruiting the immune system is critical for successful cancer treatment. Since the discovery of immune checkpoint inhibitors, immunotherapy has become the standard of care for many types of cancers. However, many patients fail to respond to immunotherapy. Further research is needed to understand the mechanisms of resistance and adjuvant therapies that can help sensitize patients to immunotherapies. Here, we will discuss how radiotherapy can change the tumor microenvironment and work synergistically with immunotherapy. We will examine different pre-clinical models focusing on their limitations and their unique advantages in studying the efficacy of treatments and the tumor microenvironment. We will also describe emerging findings from clinical trials testing the combination of immunotherapy and radiotherapy.

Temporal tracking of microglial and monocyte single-cell transcriptomics in lethal flavivirus infection

As the resident parenchymal myeloid population in the central nervous system (CNS), microglia are strategically positioned to respond to neurotropic virus invasion and have been implicated in promoting both disease resolution and progression in the acute and post-infectious phase of virus encephalitis. In a mouse model of West Nile virus encephalitis (WNE), infection of the CNS results in recruitment of large numbers of peripheral immune cells into the brain, the majority being nitric oxide (NO)-producing Ly6C^hi inflammatory monocyte-derived cells (MCs). In this model, these cells enhance immunopathology and mortality. However, the contribution of microglia to this response is currently undefined. Here we used a combination of experimental tools, including single-cell RNA sequencing (scRNA-seq), microglia and MC depletion reagents, high-dimensional spectral cytometry and computational algorithms to dissect the differential contribution of microglia and MCs to the anti-viral immune response in severe neuroinflammation seen in WNE. Intriguingly, analysis of scRNA-seq data revealed 6 unique microglia and 3 unique MC clusters that were predominantly timepoint-specific, demonstrating substantial transcriptional adaptation with disease progression over the course of WNE. While microglia and MC adopted unique gene expression profiles, gene ontology enrichment analysis, coupled with microglia and MC depletion studies, demonstrated a role for both of these cells in the trafficking of peripheral immune cells into the CNS, T cell responses and viral clearance. Over the course of infection, microglia transitioned from a homeostatic to an anti-viral and then into an immune cell-recruiting phenotype. Conversely, MC adopted antigen-presenting, immune cell-recruiting and NO-producing phenotypes, which all had anti-viral function. Overall, this study defines for the first time the single-cell transcriptomic responses of microglia and MCs over the course of WNE, demonstrating both protective and pathological roles of these cells that could potentially be targeted for differential therapeutic intervention to dampen immune-mediated pathology, while maintaining viral clearance functions.

PFKFB2-mediated glycolysis promotes lactate-driven continual efferocytosis by macrophages

Resolving-type macrophages prevent chronic inflammation by clearing apoptotic cells through efferocytosis. These macrophages are thought to rely mainly on oxidative phosphorylation, but emerging evidence suggests a possible link between efferocytosis and glycolysis. To gain further insight into this issue, we investigated molecular-cellular mechanisms involved in efferocytosis-induced macrophage glycolysis and its consequences. We found that efferocytosis promotes a transient increase in macrophage glycolysis that is dependent on rapid activation of the enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 2 (PFKFB2), which distinguishes this process from glycolysis in pro-inflammatory macrophages. Mice transplanted with activation-defective PFKFB2 bone marrow and then subjected to dexamethasone-induced thymocyte apoptosis exhibit impaired thymic efferocytosis, increased thymic necrosis, and lower expression of the efferocytosis receptors MerTK and LRP1 on thymic macrophages compared with wild-type control mice. In vitro mechanistic studies revealed that glycolysis stimulated by the uptake of a first apoptotic cell promotes continual efferocytosis through lactate-mediated upregulation of MerTK and LRP1. Thus, efferocytosis-induced macrophage glycolysis represents a unique metabolic process that sustains continual efferocytosis in a lactate-dependent manner. The differentiation of this process from inflammatory macrophage glycolysis raises the possibility that it could be therapeutically enhanced to promote efferocytosis and resolution in chronic inflammatory diseases.

Molecular profiling of TAM tyrosine kinase receptors and ligands in endometrial carcinoma: An in silico-study

OBJECTIVES

TAM Receptors (TYRO3, AXL, and MerTK) and their ligands on tumor-associated macrophages are promising therapeutic targets for most solid cancers. However, in endometrial cancer, the most common invasive gynecologic malignancy, the TAM receptor-mediated activation pathway, its molecular mechanisms, and its pathophysiology are unknown. The goal of this research; to uncover the comprehensive genetic profile of TAM receptors and ligands in endometrial cancer.

MATERIAL AND METHODS

Mutation and expression profiles of the Uterine Corpus Endometrial Carcinoma (UCEC) cohort (n = 509) were obtained using bioinformatics tools providing data from The Cancer Genome Atlas (TCGA). PolyPhen-2 and SNAP tools were used to predict the oncogenic/pathogenic properties of the identified mutations for UCEC. STRING network analysis was performed to better understand the functional relationships of the mutant proteins in cellular processes. Furthermore to the mutation profile, gene expression and survival profiles were also determined. Finally, the correlation between target genes and macrophage infiltration was investigated using the tool TIMER.

RESULTS

A total of 229 mutations were detected in 6 genes, and 81 missense mutations are pathogenic. In the UCEC cohort, the expression level of MerTK, AXL, GAS6, and PROS1 was statistically significantly lower in the patient group, while the expression level of CD47 was higher in the patient group than in the healthy group (p < 0.01). Protein-protein interaction analysis identified target genes, SRC protein responsible for important cellular mechanisms such as cell proliferation, adhesion and migration, ITGB3, ITGAV and THSB1 proteins involved in endothelial mesenchymal transition and tumor metabolism reprogramming, and FOLR1 involved in DNA replication and damage repair.

CONCLUSION

We believe that TAM receptors and their ligands may be attractive molecular targets for the treatment of endometrial carcinoma because they act as pleiotropic inhibitors of immune cells, effectively regulate phagocytic clearance of apoptotic cells, and make the tumor microenvironment a more suitable niche for the tumour.

Phosphatidylserine in the Nervous System: Cytoplasmic Regulator of the AKT and PKC Signaling Pathways and Extracellular "Eat-Me" Signal in Microglial Phagocytosis

Phosphatidylserine (PtdSer) is an important anionic phospholipid found in eukaryotic cells and has been proven to serve as a beneficial factor in the treatment of neurodegenerative diseases. PtdSer resides in the inner leaflet of the plasma membrane, where it is involved in regulating the AKT and PKC signaling pathways; however, it becomes exposed to the extracellular leaflet during neurodevelopmental processes and neurodegenerative diseases, participating in microglia-mediated synaptic and neuronal phagocytosis. In this paper, we review several characteristics of PtdSer, including the synthesis and translocation of PtdSer, the functions of cytoplasmic and exposed PtdSer, and different PtdSer-detection materials used to further understand the role of PtdSer in the nervous system.

Exploring the Expression and Function of cTyro3, a Candidate Zika Virus Receptor, in the Embryonic Chicken Brain and Inner Ear

The transmembrane protein Axl was proposed as an entry receptor for Zika virus (ZIKV) infection in vitro, but conflicting results from in vivo studies have made it difficult to establish Axl as a physiologically relevant ZIKV receptor. Both the functional redundancy of receptors and the experimental model used can lead to variable results. Therefore, it can be informative to explore alternative animal models to analyze ZIKV receptor candidates as an aid in discovering antivirals. This study used chicken embryos to examine the role of chicken Tyro3 (cTyro3), the equivalent of human Axl. Results show that endogenous cTyro3 mRNA expression overlaps with previously described hot spots of ZIKV infectivity in the brain and inner ear. We asked if ectopic expression or knockdown of cTyro3 influenced ZIKV infection in embryos. Tol2 vectors or replication-competent avian retroviruses were used in ovo to introduce full-length or truncated (presumed dominant-negative) cTyro3, respectively, into the neural tube on embryonic day two (E2). ZIKV was delivered to the brain 24 h later. cTyro3 manipulations did not alter ZIKV infection or cell death in the E5/E6 brain. Moreover, delivery of truncated cTyro3 variants to the E3 otocyst had no effect on inner ear formation on E6 or E10.

Targeted Phagocytosis Induction for Cancer Immunotherapy via Bispecific MerTK-Engaging Antibodies

The Tyro, Axl, and MerTK receptors (TAMRs) play a significant role in the clearance of apoptotic cells. In this work, the spotlight was set on MerTK, as it is one of the prominent TAMRs expressed on the surface of macrophages and dendritic cells. MerTK-specific antibodies were previously isolated from a transgenic rat-derived immune library with suitable biophysical properties. Further characterisation resulted in an agonistic MerTK antibody that led to phospho AKT activation in a dose-dependent manner. In this proof-of-concept study, a MerTK-specific antibody, MerK28, was combined with tandem, biparatopic EGFR-binding VHH camelid antibody domains (7D9G) in different architectures to generate bispecific antibodies with the capacity to bind EGFR and MerTK simultaneously. The bispecific molecules exhibited appropriate binding properties with regard to both targets in their soluble forms as well as to cells, which resulted in the engagement of macrophage-like THP-1 cells with epidermoid carcinoma A431 cells. Furthermore, targeted phagocytosis in co-culture experiments was observed only with the bispecific variants and not the parental MerTK-binding antibody. This work paves the way for the generation of bispecific macrophage-engaging antibodies for targeted phagocytosis harnessing the immune-modulating roles of MerTK in immunotherapy.

The TAM receptor tyrosine kinases Axl and Mer drive the maintenance of highly phagocytic macrophages

Many apoptotic thymocytes are generated during the course of T cell selection in the thymus, yet the machinery through which these dead cells are recognized and phagocytically cleared is incompletely understood. We found that the TAM receptor tyrosine kinases Axl and Mer, which are co-expressed by a specialized set of phagocytic thymic macrophages, are essential components of this machinery. Mutant mice lacking Axl and Mer exhibited a marked accumulation of apoptotic cells during the time that autoreactive and nonreactive thymocytes normally die. Unexpectedly, these double mutants also displayed a profound deficit in the total number of highly phagocytic macrophages in the thymus, and concomitantly exhibited diminished expression of TIM-4, CD163, and other non-TAM phagocytic engulfment systems in the macrophages that remained. Importantly, these previously unrecognized deficits were not confined to the thymus, as they were also evident in the spleen and bone marrow. They had pleiotropic consequences for the double mutants, also previously unrecognized, which included dysregulation of hemoglobin turnover and iron metabolism leading to anemia.

Tuberculosis and Autoimmunity

Tuberculosis remains a common and dangerous chronic bacterial infection worldwide. It is long-established that pathogenesis of many autoimmune diseases is mainly promoted by inadequate immune responses to bacterial agents, among them Mycobacterium tuberculosis. Tuberculosis is a multifaceted process having many different outcomes and complications. Autoimmunity is one of the processes characteristic of tuberculosis; the presence of autoantibodies was documented by a large amount of evidence. The role of autoantibodies in pathogenesis of tuberculosis is not quite clear and widely disputed. They are regarded as: (1) a result of imbalanced immune response being reactive in nature, (2) a critical part of TB pathogenicity, (3) a beginning of autoimmune disease, (4) a protective mechanism helping to eliminate microbes and infected cells, and (5) playing dual role, pathogenic and protective. There is no single autoimmunity-mechanism development in tuberculosis; different pathways may be suggested. It may be excessive cell death and insufficient clearance of dead cells, impaired autophagy, enhanced activation of macrophages and dendritic cells, environmental influences such as vitamin D insufficiency, and genetic polymorphism, both of Mycobacterium tuberculosis and host.

m6A Demethylase ALKBH5 Restrains PEDV Infection by Regulating GAS6 Expression in Porcine Alveolar Macrophages

Porcine epidemic diarrhea virus (PEDV) is a burdensome coronavirus for the global pig industry. Although its fecal-oral route has been well-recognized, increasing evidence suggests that PEDV can also spread through airborne routes, indicating that the infection may also occur in the respiratory tract. N⁶-methyladenosine (m⁶A) has been known to regulate viral replication and host immunity, yet its regulatory role and molecular mechanism regarding PEDV infection outside the gastrointestinal tract remain unexplored. In this study, we demonstrate that PEDV can infect porcine lung tissue and the 3D4/21 alveolar macrophage cell line, and the key m⁶A demethylase ALKBH5 is remarkably induced after PEDV infection. Interestingly, the disruption of ALKBH5 expression remarkably increases the infection’s capacity for PEDV. Transcriptome profiling identified dozens of putative targets of ALKBH5, including GAS6, which is known to regulate virus infectivity. Further, MeRIP-qPCR and mRNA stability analyses suggest that ALKBH5 regulates the expression of GAS6 via an m⁶A-YTHDF2-dependent mechanism. Overall, our study demonstrates that PEDV can infect porcine lung tissue and 3D4/21 cells and reveals the crucial role of ALKBH5 in restraining PEDV infections, at least partly, by influencing GAS6 through an m⁶A-YTHDF2-dependent mechanism.

Endocytic trafficking of GAS6-AXL complexes is associated with sustained AKT activation

AXL, a TAM receptor tyrosine kinase (RTK), and its ligand growth arrest-specific 6 (GAS6) are implicated in cancer metastasis and drug resistance, and cellular entry of viruses. Given this, AXL is an attractive therapeutic target, and its inhibitors are being tested in cancer and COVID-19 clinical trials. Still, astonishingly little is known about intracellular mechanisms that control its function. Here, we characterized endocytosis of AXL, a process known to regulate intracellular functions of RTKs. Consistent with the notion that AXL is a primary receptor for GAS6, its depletion was sufficient to block GAS6 internalization. We discovered that upon receptor ligation, GAS6–AXL complexes were rapidly internalized via several endocytic pathways including both clathrin-mediated and clathrin-independent routes, among the latter the CLIC/GEEC pathway and macropinocytosis. The internalization of AXL was strictly dependent on its kinase activity. In comparison to other RTKs, AXL was endocytosed faster and the majority of the internalized receptor was not degraded but rather recycled via SNX1-positive endosomes. This trafficking pattern coincided with sustained AKT activation upon GAS6 stimulation. Specifically, reduced internalization of GAS6–AXL upon the CLIC/GEEC downregulation intensified, whereas impaired recycling due to depletion of SNX1 and SNX2 attenuated AKT signaling. Altogether, our data uncover the coupling between AXL endocytic trafficking and AKT signaling upon GAS6 stimulation. Moreover, our study provides a rationale for pharmacological inhibition of AXL in antiviral therapy as viruses utilize GAS6–AXL-triggered endocytosis to enter cells.

Identification and Strategies to Mitigate High Total Clearance of Benzylamine-Substituted Biphenyl Ring Systems

For most oral small-molecule projects within drug discovery, the extent and duration of the effect are influenced by the total clearance of the compound; hence, designing compounds with low clearance remains a key focus to help enable sufficient protein target engagement. Comprehensive understanding and accurate prediction of animal clearance and pharmacokinetics provides confidence that the same can be observed for human. During a MERTK inhibitor lead optimization project, a series containing a biphenyl ring system with benzylamine meta-substitution on one phenyl and nitrogen inclusion as the meta atom on the other ring demonstrated multiple routes of compound elimination in rats. Here, we describe the identification of a structural pharmacophore involving two key interactions observed for both the MERTK program and an additional internal project. Four strategies to mitigate these clearance liabilities were identified and systematically investigated. We provide evidence that disruption of at least one of the interactions led to a significant reduction in CL that was subsequently predicted from rat hepatocytes using in vitro/in vivo extrapolation and the well-stirred scaling method. These tactics will likely be of general utility to the medicinal chemistry and DMPK community during compound optimization when similar issues are encountered for biphenyl benzylamines.

Effect of the p38 Mitogen-Activated Protein Kinase Signaling Cascade on Radiation Biodosimetry

Radiation biodosimetry based on transcriptomic analysis of peripheral blood is a valuable tool to detect radiation exposure after a radiological/nuclear event and obtain useful biological information that could predict tissue and organismal injury. However, confounding factors, including chronic inflammation or immune suppression, can potentially obscure the predictive power of the method. Members of the p38 mitogen-activated protein kinase (MAPK) family respond to pro-inflammatory signals and environmental stresses, whereas genetic ablation of the p38 signaling pathway in mice leads to reduced susceptibility to collagen-induced arthritis and experimental autoimmune encephalomyelitis that model human rheumatoid arthritis and multiple sclerosis, respectively. p38 is normally regulated by the MAP3K-MAP2K pathway in mammalian cells. However, in T cells there is an alternative pathway for p38 activation that plays an important role in antigen-receptor-activated T cells and participates in immune and inflammatory responses. To examine the role of p38 in response to radiation, we used two mouse models expressing either a p38α dominant negative (DN) mutation that globally suppresses p38 signaling or a p38αβ double-knock-in (DKI) mutant, which inhibits specifically T-cell receptor activation. We exposed p38 wild-type (p38WT) and mutant male mice to 7 Gy X rays and 24 h later whole blood was isolated subjected to whole-genome microarray and gene ontology analysis. Irradiation of p38WT mice led to a significant overrepresentation of pathways associated with morbidity and mortality, as well as organismal cell death. In contrast, these pathways were significantly underrepresented in p38DN and p38DKI mutant mice, suggesting that p38 attenuation may protect blood cells from the deleterious effects of radiation. Furthermore, radiation exposure in p38 mutant mice resulted in an enrichment of phagocytosis-related pathways, suggesting a role for p38 signaling in restricting phagocytosis of apoptotic cells after irradiation. Finally, despite the significant changes in gene expression, it was still feasible to identify a panel of genes that could accurately distinguish between irradiated and control mice, irrespective of p38 status.

Bemcentinib and Gilteritinib Inhibit Cell Growth and Impair the Endo-Lysosomal and Autophagy Systems in an AXL-Independent Manner

AXL, a receptor tyrosine kinase from the TAM (TYRO3 AXL and MER) subfamily, and its ligand growth arrest-specific 6 (GAS6) are implicated in pathogenesis of a wide array of cancers, acquisition of resistance to diverse anticancer therapies and cellular entry of viruses. The continuous development of AXL inhibitors for treatment of patients with cancer and COVID-19 underscores the need to better characterize the cellular effects of AXL targeting.

In the present study, we compared the cellular phenotypes of CRISPR-Cas9-induced depletion of AXL and its pharmacological inhibition with bemcentinib, LDC1267 and gilteritinib. Specifically, we evaluated GAS6-AXL signaling, cell viability and invasion, the endo-lysosomal system and autophagy in glioblastoma cells. We showed that depletion of AXL but not of TYRO3 inhibited GAS6-induced phosphorylation of downstream signaling effectors, AKT and ERK1/2, indicating that AXL is a primary receptor for GAS6. AXL was also specifically required for GAS6-dependent increase in cell viability but was dispensable for viability of cells grown without exogenous addition of GAS6. Furthermore, we revealed that LDC1267 is the most potent and specific inhibitor of AXL activation among the tested compounds. Finally, we found that, in contrast to AXL depletion and its inhibition with LDC1267, cell treatment with bemcentinib and gilteritinib impaired the endo-lysosomal and autophagy systems in an AXL-independent manner.

IMPLICATIONS

Altogether, our findings are of high clinical importance as we discovered that two clinically advanced AXL inhibitors, bemcentinib and gilteritinib, may display AXL-independent cellular effects and toxicity.

Erythrophagocytosis by Microglia/Macrophage in Intracerebral Hemorrhage: From Mechanisms to Translation

Intracerebral hemorrhage (ICH) is a devastating condition characterized by hematoma related mass effect. Microglia/macrophage (M φ) are rapidly recruited in order to remove the red blood cells through erythrophagocytosis. Efficient erythrophagocytosis can detoxify hemolytic products and facilitate neurological recovery after ICH. The underlying mechanisms include modulation of inflammatory response and oxidative stress, among others. It is a dynamic process mediated by a cascade of signal transduction, including “find-me” signals, “eat-me” signals and a set of phagocytotic receptors-ligand pairs that may be exploited as therapeutic targets. This review summarizes mechanistic signaling pathways of erythrophagocytosis and highlights the potential of harnessing M φ-mediated phagocytosis for ICH treatment.

The role of phosphatidylserine on the membrane in immunity and blood coagulation

The negatively charged aminophospholipid, phosphatidylserine (PtdSer), is located in the inner leaflet of the plasma membrane in normal cells, and may be exposed to the outer leaflet under some immune and blood coagulation processes. Meanwhile, Ptdser exposed to apoptotic cells can be recognized and eliminated by various immune cells, whereas on the surface of activated platelets Ptdser interacts with coagulation factors prompting enhanced production of thrombin which significantly facilitates blood coagulation. In the case where PtdSer fails in exposure or mistakenly occurs, there are occurrences of certain immunological and haematological diseases, such as the Scott syndrome and Systemic lupus erythematosus. Besides, viruses (e.g., Human Immunodeficiency Virus (HIV), Ebola virus (EBOV)) can invade host cells through binding the exposed PtdSer. Most recently, the Corona Virus Disease 2019 (COVID-19) has been similarly linked to PtdSer or its receptors. Therefore, it is essential to comprehensively understand PtdSer and its functional characteristics. Therefore, this review summarizes Ptdser, its eversion mechanism; interaction mechanism, particularly with its immune receptors and coagulation factors; recognition sites; and its function in immune and blood processes. This review illustrates the potential aspects for the underlying pathogenic mechanism of PtdSer-related diseases, and the discovery of new therapeutic strategies as well.

Up-regulated serum levels of TAM receptor tyrosine kinases in a group of Egyptian autistic children

TAM receptor family belongs to receptor tyrosine kinases (TAMRTKs). It includes three receptors; Tyro-3, Axl and Mer. TAMRTKs has a great role in resolution of inflammation due to their role in clearance of apoptotic cells by macrophages. Dysregulated TAM signaling pathways are associated with many autoimmune diseases and chronic inflammatory disorders. Autism may be an autoimmune disease in some patients. This work was the first study that investigated serum levels of the soluble ectodomain shed TAMRTKs in a group of autistic children. Serum levels of TAMRTKs were measured by ELISA in 30 autistic children aged between 3.5 and 11 years and 30 age and sex-matched healthy control children. Serum levels of TAMRTKs were significantly higher in autistic children than healthy control children (P < 0.001). Patients with severe autism had significantly higher serum levels of TAMRTKs than patients with mild to moderate autism (P < 0.01). In addition, there were significant positive correlations between scores of the Childhood Autism Rating Scale (CARS) and serum levels of TAMRTKs in autistic patients, (P < 0.01). In conclusions, serum levels of TAMRTKs were up-regulated in autistic children with significant positive correlations with the degree of the disease severity. This initial report requires further studies to investigate the relationship between TAMRTKs and autism.

Efferocytosis of vascular cells in cardiovascular disease

Cell death and the clearance of apoptotic cells are tightly regulated by various signaling molecules in order to maintain physiological tissue function and homeostasis. The phagocytic removal of apoptotic cells is known as the process of efferocytosis, and abnormal efferocytosis is linked to various health complications and diseases, such as cardiovascular disease, inflammatory diseases, and autoimmune diseases. During efferocytosis, phagocytic cells and/or apoptotic cells release signals, such as "find me" and "eat me" signals, to stimulate the phagocytic engulfment of apoptotic cells. Primary phagocytic cells are macrophages and dendritic cells; however, more recently, other neighboring cell types have also been shown to exhibit phagocytic character, including endothelial cells and fibroblasts, although they are comparatively slower in clearing dead cells. In this review, we focus on macrophage efferocytosis of vascular cells, such as endothelial cells, smooth muscle cells, fibroblasts, and pericytes, and its relation to the progression and development of cardiovascular disease. We also highlight the role of efferocytosis-related molecules and their contribution to the maintenance of vascular homeostasis.

Apoptotic Cells Trigger Calcium Entry in Phagocytes by Inducing the Orai1-STIM1 Association

Swift and continuous phagocytosis of apoptotic cells can be achieved by modulation of calcium flux in phagocytes. However, the molecular mechanism by which apoptotic cells modulate calcium flux in phagocytes is incompletely understood. Here, using biophysical, biochemical, pharmaceutical, and genetic approaches, we show that apoptotic cells induced the Orai1-STIM1 interaction, leading to store-operated calcium entry (SOCE) in phagocytes through the Mertk-phospholipase C (PLC) γ1-inositol 1,4,5-triphosphate receptor (IP₃R) axis. Apoptotic cells induced calcium release from the endoplasmic reticulum, which led to the Orai1-STIM1 association and, consequently, SOCE in phagocytes. This association was attenuated by masking phosphatidylserine. In addition, the depletion of Mertk, which indirectly senses phosphatidylserine on apoptotic cells, reduced the phosphorylation levels of PLCγ1 and IP₃R, resulting in attenuation of the Orai1-STIM1 interaction and inefficient SOCE upon apoptotic cell stimulation. Taken together, our observations uncover the mechanism of how phagocytes engulfing apoptotic cells elevate the calcium level.

AXL Receptor in Cancer Metastasis and Drug Resistance: When Normal Functions Go Askew

Simple Summary

AXL is a member of the TAM (TYRO3, AXL, MER) family of receptor tyrosine kinases. In normal physiological conditions, AXL is involved in removing dead cells and their remains, and limiting the duration of immune responses. Both functions are utilized by cancers in the course of tumour progression. Cancer cells use the AXL pathway to detect toxic environments and to activate molecular mechanisms, thereby ensuring their survival or escape from the toxic zone. AXL is instrumental in controlling genetic programs of epithelial-mesenchymal and mesenchymal-epithelial transitions, enabling cancer cells to metastasize. Additionally, AXL signaling suppresses immune responses in tumour microenvironment and thereby helps cancer cells to evade immune surveillance. The broad role of AXL in tumour biology is the reason why its inhibition sensitizes tumours to a broad spectrum of anti-cancer drugs. In this review, we outline molecular mechanisms underlying AXL function in normal tissues, and discuss how these mechanisms are adopted by cancers to become metastatic and drug-resistant.

Abstract

The TAM proteins TYRO3, AXL, and MER are receptor tyrosine kinases implicated in the clearance of apoptotic debris and negative regulation of innate immune responses. AXL contributes to immunosuppression by terminating the Toll-like receptor signaling in dendritic cells, and suppressing natural killer cell activity. In recent years, AXL has been intensively studied in the context of cancer. Both molecules, the receptor, and its ligand GAS6, are commonly expressed in cancer cells, as well as stromal and infiltrating immune cells. In cancer cells, the activation of AXL signaling stimulates cell survival and increases migratory and invasive potential. In cells of the tumour microenvironment, AXL pathway potentiates immune evasion. AXL has been broadly implicated in the epithelial-mesenchymal plasticity of cancer cells, a key factor in drug resistance and metastasis. Several antibody-based and small molecule AXL inhibitors have been developed and used in preclinical studies. AXL inhibition in various mouse cancer models reduced metastatic spread and improved the survival of the animals. AXL inhibitors are currently being tested in several clinical trials as monotherapy or in combination with other drugs. Here, we give a brief overview of AXL structure and regulation and discuss the normal physiological functions of TAM receptors, focusing on AXL. We present a theory of how epithelial cancers exploit AXL signaling to resist cytotoxic insults, in order to disseminate and relapse.

Diversity of developing peripheral glia revealed by single-cell RNA sequencing

The peripheral nervous system responds to a wide variety of sensory stimuli, a process that requires great neuronal diversity. These diverse neurons are closely associated with glial cells originating from the neural crest. However, the molecular nature and diversity among peripheral glia are not understood. Here, we used single-cell RNA sequencing to profile developing and mature glia from somatosensory dorsal root ganglia and auditory spiral ganglia. We found that glial precursors (GPs) in these two systems differ in their transcriptional profiles. Despite their unique features, somatosensory and auditory GPs undergo convergent differentiation to generate molecularly uniform myelinating and non-myelinating Schwann cells. By contrast, somatosensory and auditory satellite glial cells retain system-specific features. Lastly, we identified a glial signature gene set, providing new insights into commonalities among glia across the nervous system. This survey of gene expression in peripheral glia constitutes a resource for understanding functions of glia across different sensory modalities.

Critical Role of Synovial Tissue-Resident Macrophage and Fibroblast Subsets in the Persistence of Joint Inflammation

Rheumatoid arthritis (RA) is a chronic prototypic immune-mediated inflammatory disease which is characterized by persistent synovial inflammation, leading to progressive joint destruction. Whilst the introduction of targeted biological drugs has led to a step change in the management of RA, 30-40% of patients do not respond adequately to these treatments, regardless of the mechanism of action of the drug used (ceiling of therapeutic response). In addition, many patients who acheive clinical remission, quickly relapse following the withdrawal of treatment. These observations suggest the existence of additional pathways of disease persistence that remain to be identified and targeted therapeutically. A major barrier for the identification of therapeutic targets and successful clinical translation is the limited understanding of the cellular mechanisms that operate within the synovial microenvironment to sustain joint inflammation. Recent insights into the heterogeneity of tissue resident synovial cells, including macropahges and fibroblasts has revealed distinct subsets of these cells that differentially regulate specific aspects of inflammatory joint pathology, paving the way for targeted interventions to specifically modulate the behaviour of these cells. In this review, we will discuss the phenotypic and functional heterogeneity of tissue resident synovial cells and how this cellular diversity contributes to joint inflammation. We discuss how critical interactions between tissue resident cell types regulate the disease state by establishing critical cellular checkpoints within the synovium designed to suppress inflammation and restore joint homeostasis. We propose that failure of these cellular checkpoints leads to the emergence of imprinted pathogenic fibroblast cell states that drive the persistence of joint inflammation. Finally, we discuss therapeutic strategies that could be employed to specifically target pathogenic subsets of fibroblasts in RA.

TAM Signaling in the Nervous System

Tyro3, Axl and Mertk are members of the TAM family of tyrosine kinase receptors. TAMs are activated by two structurally homologous ligands GAS6 and PROS1. TAM receptors and ligands are widely distributed and often co-expressed in the same cells allowing diverse functions across many systems including the immune, reproductive, vascular, and the developing as well as adult nervous systems. This review will focus specifically on TAM signaling in the nervous system, highlighting the essential roles this pathway fulfills in maintaining cell survival and homeostasis, cellular functions such as phagocytosis, immunity and tissue repair. Dysfunctional TAM signaling can cause complications in development, disruptions in homeostasis which can rouse autoimmunity, neuroinflammation and neurodegeneration. The development of therapeutics modulating TAM activities in the nervous system has great prospects, however, foremost we need a complete understanding of TAM signaling pathways.

Diurnal Photoreceptor Outer Segment Renewal in Mice Is Independent of Galectin-3

Purpose

Galectin-3 (gal-3) is a soluble glycoprotein that has been associated with diverse forms of phagocytosis, including some mediated by the engulfment receptor MerTK. Retinal pigment epithelium (RPE) in vivo uses MerTK (or the related Tyro3) for phagocytosis of shed outer segment fragments during diurnal outer segment renewal. Here, we test if gal-3 plays a role in outer segment renewal in mice and if exogenous gal-3 can promote MerTK-dependent engulfment of isolated outer segment fragments by primary RPE cells in culture.

Methods

We explored age- and strain-matched wild-type (wt), lgals3^−/− and mertk^−/− mice. Immunofluorescence and immunoblotting characterized gal-3 and RPE/retina protein expression, respectively. Outer segment renewal was investigated by live imaging of phosphatidylserine (PS) exposure on photoreceptor outer segment distal tips and by microscopy of rhodopsin-labeled RPE phagosomes in tissue sections. Retinal function was assessed by recording electroretinograms (ERGs). Phagocytosis assays feeding purified outer segment fragments (POS) were conducted with added recombinant proteins testing unpassaged primary mouse RPE.

Results

Gal-3 localizes to neural retina and RPE in wt mice. The lgals3^−/− photoreceptor outer segments display normal diurnal PS exposure at distal tips. The number of rhodopsin-positive phagosomes in wt and lgals3^−/− RPE does not differ at peak or trough of diurnal phagocytosis activity. lgals3^−/− mice show light responses like wt, and their eyes contain wt levels of retinal and RPE proteins. Unlike purified protein S, recombinant gal-3 fails to promote POS engulfment by mouse primary RPE in culture.

Conclusions

Gal-3 has no essential role in MerTK-dependent outer segment renewal in mice.

The GAS6-AXL signaling pathway triggers actin remodeling that drives membrane ruffling, macropinocytosis, and cancer-cell invasion

AXL, a member of the TAM (TYRO3, AXL, MER) receptor tyrosine kinase family, and its ligand, GAS6, are implicated in oncogenesis and metastasis of many cancer types. However, the exact cellular processes activated by GAS6-AXL remain largely unexplored. Here, we identified an interactome of AXL and revealed its associations with proteins regulating actin dynamics. Consistently, GAS6-mediated AXL activation triggered actin remodeling manifested by peripheral membrane ruffling and circular dorsal ruffles (CDRs). This further promoted macropinocytosis that mediated the internalization of GAS6-AXL complexes and sustained survival of glioblastoma cells grown under glutamine-deprived conditions. GAS6-induced CDRs contributed to focal adhesion turnover, cell spreading, and elongation. Consequently, AXL activation by GAS6 drove invasion of cancer cells in a spheroid model. All these processes required the kinase activity of AXL, but not TYRO3, and downstream activation of PI3K and RAC1. We propose that GAS6-AXL signaling induces multiple actin-driven cytoskeletal rearrangements that contribute to cancer-cell invasion.

Coagulation signaling and cancer immunotherapy

The last decades have delineated many interactions of the hemostatic system with cancer cells that are pivotal for cancer-associated thrombosis, angiogenesis and metastasis. Expanding evidence shows that platelets, the tissue factor pathway, and proteolytic signaling involving protease-activated receptors (PARs) are also central players in innate and adaptive immunity. Recent studies in immune-competent mice have uncovered new immune-evasive roles of coagulation signaling networks in the development and growth of different preclinical tumor models. Tumor-type specific PAR1 signaling facilitates the escape from immune surveillance by cytotoxic T cells. In addition, tumor-associated macrophages produce factor X (FX) and cell autonomous FXa-PAR2 signaling emerges as a central mechanism for tumor-promoting macrophage polarization in the tumor microenvironment. Pharmacological targeting of this signaling pathway with tissue penetrating oral FXa inhibitor reprograms macrophage phenotypes, enhances tumor antigen presentation, and expands tumor-killing cytotoxic lymphocytes. Importantly, by specifically targeting innate immune cells, the oral FXa inhibitor rivaroxaban synergizes with checkpoint inhibitor therapy in enhancing antigen-specific antitumor immunity. In similar experiments, anticoagulation with heparin is inefficient to block extravascular coagulation signaling. Thus, antithrombotic therapy with oral FXa inhibitors may contribute to reversing tumor immune-evasive mechanisms and enhance the clinical outcome of targeted immuno-therapy regimens.

Myeloid cell-derived PROS1 inhibits tumor metastasis by regulating inflammatory and immune responses via IL-10

Stimulation of TAM (TYRO3, AXL, and MERTK) receptor tyrosine kinases promotes tumor progression through numerous cellular mechanisms. TAM cognate ligands GAS6 and PROS1 (for TYRO3 and MERTK) are secreted by host immune cells, an interaction which may support tumor progression. Here, we revealed an unexpected antimetastatic role for myeloid-derived PROS1: suppressing metastatic potential in lung and breast tumor models. Pros1 deletion in myeloid cells led to increased lung metastasis, independent of primary tumor infiltration. PROS1-cKO bone marrow-derived macrophages (BMDMs) led to elevated TNF-α, IL-6, Nos2, and IL-10 via modulation of the Socs3/NF-κB pathway. Conditioned medium from cKO BMDMs enhanced EMT, ERK, AKT, and STAT3 activation within tumor cells and promoted IL-10-dependent invasion and survival. Macrophages isolated from metastatic lungs modulated T cell proliferation and function, as well as expression of costimulatory molecules on DCs in a PROS1-dependent manner. Inhibition of MERTK kinase activity blocked PROS1-mediated suppression of TNF-α and IL-6 but not IL-10. Overall, using lung and breast cancer models, we identified the PROS1/MERTK axis within BMDMs as a potent regulator of adaptive immune responses with a potential to suppress metastatic seeding and revealed IL-10 regulation by PROS1 to deviate from that of TNF-α and IL-6.

Chikungunya and Zika Viruses: Co-Circulation and the Interplay between Viral Proteins and Host Factors

Chikungunya and Zika viruses, both transmitted by mosquito vectors, have globally re-emerged over for the last 60 years and resulted in crucial social and economic concerns. Presently, there is no specific antiviral agent or vaccine against these debilitating viruses. Understanding viral-host interactions is needed to develop targeted therapeutics. However, there is presently limited information in this area. In this review, we start with the updated virology and replication cycle of each virus. Transmission by similar mosquito vectors, frequent co-circulation, and occurrence of co-infection are summarized. Finally, the targeted host proteins/factors used by the viruses are discussed. There is an urgent need to better understand the virus-host interactions that will facilitate antiviral drug development and thus reduce the global burden of infections caused by arboviruses.

Vitamin K-Dependent γ-Glutamyl Carboxylase in Sertoli Cells Is Essential for Male Fertility in Mice

γ-Glutamyl carboxylase (GGCX) is a vitamin K (VK)-dependent enzyme that catalyzes the γ-carboxylation of glutamic acid residues in VK-dependent proteins. The anticoagulant warfarin is known to reduce GGCX activity by inhibiting the VK cycle and was recently shown to disrupt spermatogenesis. To explore GGCX function in the testis, here, we generated Sertoli cell-specific Ggcx conditional knockout (Ggcx scKO) mice and investigated their testicular phenotype. Ggcx scKO mice exhibited late-onset male infertility. They possessed morphologically abnormal seminiferous tubules containing multinucleated and apoptotic germ cells, and their sperm concentration and motility were substantially reduced. The localization of connexin 43 (Cx43), a gap junction protein abundantly expressed in Sertoli cells and required for spermatogenesis, was distorted in Ggcx scKO testes, and Cx43 overexpression in Sertoli cells rescued the infertility of Ggcx scKO mice. These results highlight GGCX activity within Sertoli cells, which promotes spermatogenesis by regulating the intercellular connection between Sertoli cells and germ cells.

Maintaining homeostatic control of periodontal epithelial tissue

Years of coevolution with resident microbes has made them an essential component of health. Yet, little is known about oral commensal bacteria's contribution to and role in the maintenance of oral health and homeostasis. Commensal bacteria are speculated to play a host protective role in the maintenance of health. In this review, we describe and provide examples of the coordinate regulation that occurs between oral commensal bacteria and the host innate immune response to modulate and maintain oral homeostasis.

Electro-acupuncture Suppresses AXL Expression in Dorsal Root Ganglion Neurons and Enhances Analgesic Effect of AXL Inhibitor in Spinal Nerve Ligation Induced-Neuropathic Pain Rats

Electro-acupuncture (EA) has been used for clinic analgesia for many years. However, its mechanisms are not fully understood. We recently reported that AXL, a tyrosine kinase receptor, contributes to the peripheral mechanism of neuropathic pain. We here aim to figure out the significance of EA on neuropathic pain mediated by AXL in dorsal root ganglion (DRG). Spinal nerve ligation (SNL) was used as a neuropathic pain model. EA was applied at ''Huantiao'' (GB-30) and ''Yanglingquan'' (GB-34) acupoints for 30 min daily from day 7 to day 10 after SNL. EA not only gradually attenuated SNL-induced mechanical allodynia, but also suppressed the expression of phosphorylated AXL (p-AXL) and AXL in injured DRGs of SNL rats examined by western blotting and immunofluorescence. Moreover, intrathecal injection of the subthreshold dose of AXL inhibitor TP0903, significantly prolonged the analgesic time of single EA treatment and enhanced the analgesic effect of repeated EA treatments, suggesting a synergic effect of EA and AXL inhibitor. These results indicate that AXL signaling underlies EA analgesia and combination of AXL inhibitor and EA might be a new strategy for clinic analgesia on neuropathic pain.

Apoptotic Donor Cells in Transplantation

Despite significant advances in prevention and treatment of transplant rejection with immunosuppressive medications, we continue to face challenges of long-term graft survival, detrimental medication side effects to both the recipient and transplanted organ together with risks for opportunistic infections. Transplantation tolerance has so far only been achieved through hematopoietic chimerism, which carries with it a serious and life-threatening risk of graft versus host disease, along with variability in persistence of chimerism and uncertainty of sustained tolerance. More recently, numerous in vitro and in vivo studies have explored the therapeutic potential of silent clearance of apoptotic cells which have been well known to aid in maintaining peripheral tolerance to self. Apoptotic cells from a donor not only have the ability of down regulating the immune response, but also are a way of providing donor antigens to recipient antigen-presenting-cells that can then promote donor-specific peripheral tolerance. Herein, we review both laboratory and clinical evidence that support the utility of apoptotic cell-based therapies in prevention and treatment of graft versus host disease and transplant rejection along with induction of donor-specific tolerance in solid organ transplantation. We have highlighted the potential limitations and challenges of this apoptotic donor cell-based therapy together with ongoing advancements and attempts made to overcome them.

Serum Gas6 and Axl as non-invasive biomarkers of advanced histological stage in primary biliary cholangitis

AIM

Advanced histological stage is an important factor in individual risk stratification in patients with primary biliary cholangitis (PBC). Non-invasive biomarkers for advanced histological stage are needed. We assessed the utility of Gas6 and Axl as biomarkers for advanced histological stage in patients with PBC.

METHODS

A total of 113 biopsy-proven PBC patients and 20 healthy controls were included in this study. Serum Axl and Gas6 were measured using enzyme-linked immunosorbent assay. The Gas6 / albumin ratio and Axl / albumin ratio were also evaluated as biomarkers of histological stage.

RESULTS

Serum Axl (42.6 ng/mL vs. 30.6 ng/mL, P < 0.001) and Gas6 (21.1 ng/mL vs. 18.8 ng/mL, P = 0.007) levels in PBC patients were significantly higher than those in healthy controls. The Axl / albumin ratio was 10.4, and the Gas6 / albumin ratio was 7.6 in patients with PBC. Gas6 and Axl were significantly correlated with aspartate aminotransferase, bilirubin, albumin, and platelets. Gas6 and Axl levels in patients with an advanced Scheuer stage and an advanced Nakanuma stage were significantly higher than those in other patients. The area under the receiver operating characteristic curve (AUROC) of Axl, Gas6, Axl / albumin, and Gas6 / albumin for diagnosing Scheuer stage 4 was 0.733, 0.837, 0.845, and 0.893, respectively. The AUROC of Axl, Gas6, Axl / albumin, and Gas6 / albumin for diagnosing Nakanuma stage 4 was 0.794, 0.834, 0.869, and 0.898, respectively.

CONCLUSION

High levels of Gas6 and Axl were associated with advanced histological stage in PBC patients. Furthermore, the Gas6 / albumin ratio and the Axl / albumin ratio showed a high AUROC for diagnosing advanced histological stage.

Recent advancements in role of TAM receptors on efferocytosis, viral infection, autoimmunity, and tissue repair

Evolutionarily conserved highly regulated process of apoptosis has been a major physiological process throughout the entire evolutionary history of living beings that has impacted the process of evolution itself. One of the key features of this highly researched field of science is the process of phosphatidylserine (PS) externalization by the different membrane bound enzymes. The process is a result of series of biological events and is associated with various biological outcomes depending on the proper recognition of this ligand. In this review, we will briefly summarize the recent advancement in the field pertaining to the set of receptors, known as TAM (Tyro3, Axl and Mertk) receptors, for their influence in the recognition of various PS externalization events and mediation of pathological outcomes such as autoimmunity, cancer, and tissue repair.

Immunological role of TAM receptors in the cancer microenvironment

TAM receptors belong to the family of receptor tyrosine kinases, comprising of Tyro3, Axl and Mertk receptors (TAMs) and are important homeostatic regulators of inflammation in higher eukaryotes. Along with their ligands, Gas6 and ProteinS, TAMs acts as receptors to phosphatidylserine (PtdSer), an anionic phospholipid that becomes externalized on the surface of apoptotic and stressed cells. TAM receptors, specially Mertk, have been well established to play a role in the process of efferocytosis, the engulfment of dying cells. Besides being efferocytic receptors, TAMs are pleiotropic immune modulators as the lack of TAM receptors in various mouse models lead to chronic inflammation and autoimmunity. Owing to their immune modulatory role, the PtdSer-TAM receptor signaling axis has been well characterized as a global immune-suppressive signal, and in cancers, and emerging literature implicates TAM receptors in cancer immunology and anti-tumor therapeutics. In the tumor microenvironment, immune-suppressive signals, such as ones that originate from TAM receptor signaling can be detrimental to anti-tumor therapy. In this chapter, we discuss immune modulatory functions of TAM receptors in the tumor microenvironment as well role of differentially expressed TAM receptors and their interactions with immune and tumor cells. Finally, we describe current strategies being utilized for targeting TAMs in several cancers and their implications in immunotherapy.