Mertk deficiency affects macrophage directional migration via disruption of cytoskeletal organization

Myelin debris phagocytosis in demyelinating disease

Demyelinating diseases are often caused by a variety of triggers, including immune responses, viral infections, malnutrition, hypoxia, or genetic factors, all of which result in the loss of myelin in the nervous system. The accumulation of myelin debris at the lesion site leads to neuroinflammation and inhibits remyelination; therefore, it is crucial to promptly remove the myelin debris. Initially, Fc and complement receptors on cellular surfaces were the primary clearance receptors responsible for removing myelin debris. However, subsequent studies have unveiled the involvement of additional receptors, including Mac-2, TAM receptors, and the low-density lipoprotein receptor-related protein 1, in facilitating the removal process. In addition to microglia and macrophages, which serve as the primary effector cells in the disease phase, a variety of other cell types such as astrocytes, Schwann cells, and vascular endothelial cells have been demonstrated to engage in the phagocytosis of myelin debris. Furthermore, we have concluded that oligodendrocyte precursor cells, as myelination precursor cells, also exhibit this phagocytic capability. Moreover, our research group has innovatively identified the low-density lipoprotein receptor as a potential phagocytic receptor for myelin debris. In this article, we discuss the functional processes of various phagocytes in demyelinating diseases. We also highlight the alterations in signaling pathways triggered by phagocytosis, and provide a comprehensive overview of the various phagocytic receptors involved. Such insights are invaluable for pinpointing potential therapeutic strategies for the treatment of demyelinating diseases by targeting phagocytosis.

The emerging relationship between vitamin K and neurodegenerative diseases: a review of current evidence

Neurodegenerative disease refers to a group of disorders that predominantly damage the neurons in the brain. Despite significant progress in the knowledge of neurodegenerative diseases, there is currently no disease-modifying drug available. Vitamin K was first established for its involvement in blood clotting, but there is now compelling evidence indicating its role in the neurological system. In particular, the results of recent studies on the effects of vitamin K2 on preventing apoptosis, oxidative stress, and microglial activation in neuron cells through its role in electron transport are very promising against Alzheimer's disease. In addition to its protective effect on cognitive functions, its inhibitory effects on inflammation and α-synuclein fibrillization in Parkinson's disease, which has been revealed in recent years, are remarkable. Although there are many studies on the mechanism and possible treatment methods of neurodegenerative diseases, especially Parkinson's and Alzheimer's disease, studies on the relationship between vitamin K and neurodegenerative diseases are very limited, yet have promising findings. Vitamin K has also been proposed for therapeutic use in multiple sclerosis patients to lower the intensity or to slow down the progression of the disease. Accordingly, the aim of this study is to review the current evidence for the use of vitamin K supplementation in neurodegenerative diseases, in particular Alzheimer's disease, Parkinson's disease, and multiple sclerosis.

Regulation of bone homeostasis by MERTK and TYRO3

The fine equilibrium of bone homeostasis is maintained by bone-forming osteoblasts and bone-resorbing osteoclasts. Here, we show that TAM receptors MERTK and TYRO3 exert reciprocal effects in osteoblast biology: Osteoblast-targeted deletion of MERTK promotes increased bone mass in healthy mice and mice with cancer-induced bone loss, whereas knockout of TYRO3 in osteoblasts shows the opposite phenotype. Functionally, the interaction of MERTK with its ligand PROS1 negatively regulates osteoblast differentiation via inducing the VAV2-RHOA-ROCK axis leading to increased cell contractility and motility while TYRO3 antagonizes this effect. Consequently, pharmacologic MERTK blockade by the small molecule inhibitor R992 increases osteoblast numbers and bone formation in mice. Furthermore, R992 counteracts cancer-induced bone loss, reduces bone metastasis and prolongs survival in preclinical models of multiple myeloma, breast- and lung cancer. In summary, MERTK and TYRO3 represent potent regulators of bone homeostasis with cell-type specific functions and MERTK blockade represents an osteoanabolic therapy with implications in cancer and beyond.

Synaptic pruning of murine adult-born neurons by microglia depends on phosphatidylserine

New neurons, continuously added in the adult olfactory bulb (OB) and hippocampus, are involved in information processing in neural circuits. Here, we show that synaptic pruning of adult-born neurons by microglia depends on phosphatidylserine (PS), whose exposure on dendritic spines is inversely correlated with their input activity. To study the role of PS in spine pruning by microglia in vivo, we developed an inducible transgenic mouse line, in which the exposed PS is masked by a dominant-negative form of milk fat globule-EGF-factor 8 (MFG-E8), MFG-E8D89E. In this transgenic mouse, the spine pruning of adult-born neurons by microglia is impaired in the OB and hippocampus. Furthermore, the electrophysiological properties of these adult-born neurons are altered in MFG-E8D89E mice. These data suggest that PS is involved in the microglial spine pruning and the functional maturation of adult-born neurons. The MFG-E8D89E-based genetic approach shown in this study has broad applications for understanding the biology of PS-mediated phagocytosis in vivo.

Impact of SOCE Abolition by ORAI1 Knockout on the Proliferation, Adhesion, and Migration of HEK-293 Cells

Store-operated calcium entry (SOCE) provided through channels formed by ORAI proteins is a major regulator of several cellular processes. In immune cells, it controls fundamental processes such as proliferation, cell adhesion, and migration, while in cancer, SOCE and ORAI1 gene expression are dysregulated and lead to abnormal migration and/or cell proliferation. In the present study, we used the CRISPR/Cas9 technique to delete the ORAI1 gene and to identify its role in proliferative and migrative properties of the model cell line HEK-293. We showed that ORAI1 deletion greatly reduced SOCE. Thereby, we found that this decrease and the absence of ORAI1 protein did not affect HEK-293 proliferation. In addition, we determined that ORAI1 suppression did not affect adhesive properties but had a limited impact on HEK-293 migration. Overall, we showed that ORAI1 and SOCE are largely dispensable for cellular proliferation, migration, and cellular adhesion of HEK-293 cells. Thus, despite its importance in providing Ca2+ entry in non-excitable cells, our results indicate that the lack of SOCE does not deeply impact HEK-293 cells. This finding suggests the existence of compensatory mechanism enabling the maintenance of their physiological function.

MERTK Inhibition: Potential as a Treatment Strategy in EGFR Tyrosine Kinase Inhibitor-Resistant Non-Small Cell Lung Cancer

Epidermal growth factor tyrosine kinase inhibitors (EGFR-TKIs) are currently the most effective treatment for non-small cell lung cancer (NSCLC) patients, who carry primary EGFR mutations. However, the patients eventually develop drug resistance to EGFR-TKIs after approximately one year. In addition to the acquisition of the EGFR T790M mutation, the activation of alternative receptor-mediated signaling pathways is a common mechanism for conferring the insensitivity of EGFR-TKI in NSCLC. Upregulation of the Mer receptor tyrosine kinase (MERTK), which is a member of the Tyro3-Axl-MERTK (TAM) family, is associated with a poor prognosis of many cancers. The binding of specific ligands, such as Gas6 and PROS1, to MERTK activates phosphoinositide 3-kinase (PI3K)/Akt and mitogen-activated protein kinase (MAPK) cascades, which are the signaling pathways shared by EGFR. Therefore, the inhibition of MERTK can be considered a new therapeutic strategy for overcoming the resistance of NSCLC to EGFR-targeted agents. Although several small molecules and monoclonal antibodies targeting the TAM family are being developed and have been described to enhance the chemosensitivity and converse the resistance of EGFR-TKI, few have specifically been developed as MERTK inhibitors. The further development and investigation of biomarkers which can accurately predict MERTK activity and the response to MERTK inhibitors and MERTK-specific drugs are vitally important for obtaining appropriate patient stratification and increased benefits in clinical applications.

MERTK in cancer therapy: Targeting the receptor tyrosine kinase in tumor cells and the immune system

The receptor tyrosine kinase MERTK is aberrantly expressed in numerous human malignancies, and is a novel target in cancer therapeutics. Physiologic roles of MERTK include regulation of tissue homeostasis and repair, innate immune control, and platelet aggregation. However, aberrant expression in a wide range of liquid and solid malignancies promotes neoplasia via growth factor independence, cell cycle progression, proliferation and tumor growth, resistance to apoptosis, and promotion of tumor metastases. Additionally, MERTK signaling contributes to an immunosuppressive tumor microenvironment via induction of an anti-inflammatory cytokine profile and regulation of the PD-1 axis, as well as regulation of macrophage, myeloid-derived suppressor cell, natural killer cell and T cell functions. Various MERTK-directed therapies are in preclinical development, and clinical trials are underway. In this review we discuss MERTK inhibition as an emerging strategy for cancer therapy, focusing on MERTK expression and function in neoplasia and its role in mediating resistance to cytotoxic and targeted therapies as well as in suppressing anti-tumor immunity. Additionally, we review preclinical and clinical pharmacological strategies to target MERTK.

TAM receptors, Phosphatidylserine, inflammation, and Cancer

Abstract

The numerous and diverse biological roles of Phosphatidylserine (PtdSer) are featured in this special issue. This review will focus on PtdSer as a cofactor required for stimulating TYRO3, AXL and MERTK – comprising the TAM family of receptor tyrosine kinases by their ligands Protein S (PROS1) and growth-arrest-specific 6 (GAS6) in inflammation and cancer. As PtdSer binding to TAMs is a requirement for their activation, the biological repertoire of PtdSer is now recognized to be broadened to include functions performed by TAMs. These include key homeostatic roles necessary for preserving a healthy steady state in different tissues, controlling inflammation and further additional roles in diseased states and cancer. The impact of PtdSer on inflammation and cancer through TAM signaling is a highly dynamic field of research. This review will focus on PtdSer as a necessary component of the TAM receptor-ligand complex, and for maximal TAM signaling. In particular, interactions between tumor cells and their immediate environment - the tumor microenvironment (TME) are highlighted, as both cancer cells and TME express TAMs and secrete their ligands, providing a nexus for a multifold of cross-signaling pathways which affects both immune cells and inflammation as well as tumor cell biology and growth. Here, we will highlight the current and emerging knowledge on the implications of PtdSer on TAM signaling, inflammation and cancer.

Graphical Abstract

ZIKV Strains Differentially Affect Survival of Human Fetal Astrocytes versus Neurons and Traffic of ZIKV-Laden Endocytotic Compartments

Malformations of the fetal CNS, known as microcephaly, have been linked to Zika virus (ZIKV) infection. Here, the responses of mammalian and mosquito cell lines, in addition to primary human fetal astrocytes and neurons were studied following infection by ZIKV strains Brazil 2016 (ZIKV-BR), French Polynesia 2013 (ZIKV-FP), and Uganda #976 1947 (ZIKV-UG). Viral production, cell viability, infectivity rate, and mobility of endocytotic ZIKV-laden vesicles were compared. All cell types (SK-N-SH, Vero E6, C6/36, human fetal astrocytes and human fetal neurons) released productive virus. Among primary cells, astrocytes were more susceptible to ZIKV infection than neurons, released more progeny virus and tolerated higher virus loads than neurons. In general, the infection rate of ZIKV-UG strain was the highest. All ZIKV strains elicited differences in trafficking of ZIKV-laden endocytotic vesicles in the majority of cell types, including astrocytes and neurons, except in mosquito cells, where ZIKV infection failed to induce cell death. These results represent a thorough screening of cell viability, infection and production of three ZIKV strains in five different cell types and demonstrate that ZIKV affects vesicle mobility in all but mosquito cells.

A simple culture method for liver and intestinal tissue-resident macrophages from neonatal mice

The liver and intestine contain a remarkably large portion of tissue-resident macrophage cells representing a phenotype that downregulates inflammation and initiates tissue repair. Here, liver and intestinal tissues obtained from neonatal mice were minced, enzymatically digested, and incubated in RPMI1640-based media. In a 2-wk culture, spherical floating cells emerged on a fibroblastic sheet. These cells showed phagocytic activity and F4/80⁺-CD11b⁺-CD206⁺-Arg1⁺-iNOS^--CD209a^- phenotype, suggesting that these cells are tissue-resident macrophages. These macrophages proliferated in the co-culture system in the presence of fibroblastic feeder cell layer and absence of supplemental cytokines; the co-culture system did not cause a significant change in the phenotype of cells grown in a 4-wk culture. On the feeder cells, macrophage density was approximately 1.5 × 10⁴/cm² and the doubling time was approximately 70 h. Based on these observations, we present a simple method for the isolation and propagation of tissue-resident macrophages resembling M2 macrophage from neonatal mice, and this method provides a useful platform for in vitro studies of tissue-resident macrophages.

The Relationships Between Vitamin K and Cognition: A Review of Current Evidence

Vitamin K is a fat-soluble nutrient discovered in 1935 and its role in blood coagulation has been thoroughly explored. In recent years, studies conducted in vitro and on animals highlighted vitamin K involvement in brain cells development and survival. In particular, vitamin K seems to have an antiapoptotic and anti-inflammatory effect mediated by the activation of Growth Arrest Specific Gene 6 and Protein S. Moreover, this vitamin is involved in sphingolipids metabolism, a class of lipids that participate in the proliferation, differentiation, and survival of brain cells. An altered expression in sphingolipids profile has been related to neuroinflammation and neurodegeneration. This review stems from a growing interest in the role of vitamin K in brain functions, especially in cognition, also in view of an expected increase of prevalence of Alzheimer's disease and other forms of dementia. It collects recent researches that show interesting, even though not definitive, evidence of a direct correlation between vitamin K levels and cognitive performance. Moreover, vitamin K antagonists, used worldwide as oral anticoagulants, according to recent studies may have a negative influence on cognitive domains such as visual memory, verbal fluency and brain volume. The aim of this review is to analyze the evidence of clinical studies carried out up to date on the relationship between vitamin K intake and cognitive performances. The involvement of vitamin K antagonists (VKAs) in declining cognitive performances is also addressed separately.

Estramustine Phosphate Inhibits TGF-β-Induced Mouse Macrophage Migration and Urokinase-Type Plasminogen Activator Production

Transforming growth factor-beta (TGF-β) has been demonstrated as a key regulator of immune responses including monocyte/macrophage functions. TGF-β regulates macrophage cell migration and polarization, as well as it is shown to modulate macrophage urokinase-type plasminogen activator (uPA) production, which also contributes to macrophage chemotaxis and migration toward damaged or inflamed tissues. Microtubule (MT) cytoskeleton dynamic plays a key role during the cell motility, and any interference on the MT network profoundly affects cell migration. In this study, by using estramustine phosphate (EP), which modifies MT stability, we analysed whether tubulin cytoskeleton contributes to TGF-β-induced macrophage cell migration and uPA expression. We found out that, in the murine macrophage cell line RAW 264.7, EP at noncytotoxic concentrations inhibited cell migration and uPA expression induced by TGF-β. Moreover, EP greatly reduced the capacity of TGF-β to trigger the phosphorylation and activation of its downstream Smad3 effector. Furthermore, Smad3 activation seems to be critical for the increased cell motility. Thus, our data suggest that EP, by interfering with MT dynamics, inhibits TGF-β-induced RAW 264.7 cell migration paralleled with reduction of uPA induction, in part by disabling Smad3 activation by TGF-β.

Cardiac macrophage biology in the steady-state heart, the aging heart, and following myocardial infarction

Macrophages play critical roles in homeostatic maintenance of the myocardium under normal conditions and in tissue repair after injury. In the steady-state heart, resident cardiac macrophages remove senescent and dying cells and facilitate electrical conduction. In the aging heart, the shift in macrophage phenotype to a proinflammatory subtype leads to inflammaging. Following myocardial infarction (MI), macrophages recruited to the infarct produce both proinflammatory and anti-inflammatory mediators (cytokines, chemokines, matrix metalloproteinases, and growth factors), phagocytize dead cells, and promote angiogenesis and scar formation. These diverse properties are attributed to distinct macrophage subtypes and polarization status. Infarct macrophages exhibit a proinflammatory M1 phenotype early and become polarized toward an anti-inflammatory M2 phenotype later post-MI. Although this classification system is oversimplified and needs to be refined to accommodate the multiple different macrophage subtypes that have been recently identified, general concepts on macrophage roles are independent of subtype classification. This review summarizes current knowledge about cardiac macrophage origins, roles, and phenotypes in the steady state, with aging, and after MI, as well as highlights outstanding areas of investigation.

The Gas6/TAM System and Multiple Sclerosis

Growth arrest specific 6 (Gas6) is a multimodular circulating protein, the biological actions of which are mediated by the interaction with three transmembrane tyrosine kinase receptors: Tyro3, Axl, and MerTK, collectively named TAM. Over the last few decades, many progresses have been done in the understanding of the biological activities of this highly pleiotropic system, which plays a role in the regulation of immune response, inflammation, coagulation, cell growth, and clearance of apoptotic bodies. Recent findings have further related Gas6 and TAM receptors to neuroinflammation in general and, specifically, to multiple sclerosis (MS). In this paper, we review the biology of the Gas6/TAM system and the current evidence supporting its potential role in the pathogenesis of MS.

TAM receptors regulate multiple features of microglial physiology

Microglia are damage sensors for the central nervous system (CNS), and the phagocytes responsible for the routine non-inflammatory clearance of dead brain cells¹. Here we show that the TAM receptor tyrosine kinases Mer and Axl² regulate these microglial functions. We find that mice deficient in microglial Mer and Axl exhibit a marked accumulation of apoptotic cells (ACs) specifically in neurogenic regions of the adult CNS, and that microglial phagocytosis of the ACs generated during adult neurogenesis^3,4 is normally driven by both TAM receptor ligands – Gas6 and Protein S⁵. Live two-photon imaging demonstrates that the microglial response to brain damage is also TAM-regulated, as TAM-deficient microglia display reduced process motility and delayed convergence to sites of injury. Finally, we show that microglial expression of Axl is prominently up-regulated in the inflammatory environment that develops in a mouse model of Parkinson’s disease⁶. Together, these results establish TAM receptors as both controllers of microglial physiology and potential targets for therapeutic intervention in CNS disease.

Software-assisted morphometric and phenotype analyses of human peripheral blood monocyte-derived macrophages induced by a microenvironment model of human esophageal squamous cell carcinoma

Human macrophages play important roles in tumor promotion and are called tumor-associated macrophages (TAMs). We previously demonstrated that human esophageal squamous cell carcinomas (ESCCs) contain TAMs and that these TAMs tend to have tumor-supporting features. Here we exposed human macrophages to conditioned media of TE-series human ESCC cell lines (TECMs) to generate an ESCC extracellular stimuli-influenced TAM model. CD14(+) peripheral blood monocytes (PBMos) from healthy donors were treated with M-CSF and with additional IL-4 or TECM exposure. Morphological changes of the cells and the induction of CD163/CD204 proteins were detected in the TECM-exposed model TAMs by immunofluorescence. A software-assisted immunofluorescent cell image analysis showed increased CD163/CD204 positivity in the model TAMs and a weak to moderate positive correlation between the cytoplasmic area and the sum fluorescent intensity of CD204. Morphological changes of the cells were significantly reflected by several cytomorphometric parameters. PBMos were elongated with M-CSF treatment, then enlarged with TECM exposure. The cytoplasmic aspect ratio was decreased by M-CSF treatment and slightly increased by TECM exposure. The nuclear-cytoplasmic ratio decreased during the whole process of cell differentiation. This system is useful for quantitative assessments of TAM-like morphological changes of macrophages and the induction of CD163/CD204 in a model ESCC microenvironment.

Cardiomyocytes induce macrophage receptor shedding to suppress phagocytosis

BACKGROUND

Mobilization of the innate immune response to clear and metabolize necrotic and apoptotic cardiomyocytes is a prerequisite to heart repair after cardiac injury. Suboptimal kinetics of dying myocyte clearance leads to secondary necrosis, and in the case of the heart, increased potential for collateral loss of neighboring non-regenerative myocytes. Despite the importance of myocyte phagocytic clearance during heart repair, surprisingly little is known about its underlying cell and molecular biology.

OBJECTIVE

To determine if phagocytic receptor MERTK is expressed in human hearts and to elucidate key sequential steps and phagocytosis efficiency of dying adult cardiomyocytes, by macrophages.

RESULTS

In infarcted human hearts, expression profiles of the phagocytic receptor MER-tyrosine kinase (MERTK) mimicked that found in experimental ischemic mouse hearts. Electron micrographs of myocardium identified MERTK signal along macrophage phagocytic cups and Mertk-/- macrophages contained reduced digested myocyte debris after myocardial infarction. Ex vivo co-culture of primary macrophages and adult cardiomyocyte apoptotic bodies revealed reduced engulfment relative to resident cardiac fibroblasts. Inefficient clearance was not due to the larger size of myocyte apoptotic bodies, nor were other key steps preceding the formation of phagocytic synapses significantly affected; this included macrophage chemotaxis and direct binding of phagocytes to myocytes. Instead, suppressed phagocytosis was directly associated with myocyte-induced inactivation of MERTK, which was partially rescued by genetic deletion of a MERTK proteolytic susceptibility site.

CONCLUSION

Utilizing an ex vivo co-cultivation approach to model key cellular and molecular events found in vivo during infarction, cardiomyocyte phagocytosis was found to be inefficient, in part due to myocyte-induced shedding of macrophage MERTK. These findings warrant future studies to identify other cofactors of macrophage-cardiomyocyte cross-talk that contribute to cardiac pathophysiology.