Molecular uncoupling of fractalkine-mediated cell adhesion and signal transduction. Rapid flow arrest of CX3CR1-expressing cells is independent of G-protein activation

Unraveling the enigma: housekeeping gene Ugt1a7c as a universal biomarker for microglia

Background

Microglia, brain resident macrophages, play multiple roles in maintaining homeostasis, including immunity, surveillance, and protecting the central nervous system through their distinct activation processes. Identifying all types of microglia-driven populations is crucial due to the presence of various phenotypes that differ based on developmental stages or activation states. During embryonic development, the E8.5 yolk sac contains erythromyeloid progenitors that go through different growth phases, eventually resulting in the formation of microglia. In addition, microglia are present in neurological diseases as a diverse population. So far, no individual biomarker for microglia has been discovered that can accurately identify and monitor their development and attributes.

Summary

Here, we highlight the newly defined biomarker of mouse microglia, UGT1A7C, which exhibits superior stability in expression during microglia development and activation compared to other known microglia biomarkers. The UGT1A7C sensing chemical probe labels all microglia in the 3xTG AD mouse model. The expression of Ugt1a7c is stable during development, with only a 4-fold variation, while other microglia biomarkers, such as Csf1r and Cx3cr1, exhibit at least a 10-fold difference. The UGT1A7C expression remains constant throughout its lifespan. In addition, the expression and activity of UGT1A7C are the same in response to different types of inflammatory activators' treatment in vitro.

Conclusion

We propose employing UGT1A7C as the representative biomarker for microglia, irrespective of their developmental state, age, or activation status. Using UGT1A7C can reduce the requirement for using multiple biomarkers, enhance the precision of microglia analysis, and even be utilized as a standard for gene/protein expression.

Fractalkine Signalling (CX3CL1/CX3CR1 Axis) as an Emerging Target in Coronary Artery Disease

Acute myocardial infarction (MI) is the most common and dramatic complication of atherosclerosis, which, despite successful reperfusion therapy, can lead to incident heart failure (HF). HF occurs when the healing process is impaired due to adverse left ventricular remodelling, and can be the result of so-called ischaemia/reperfusion injury (IRI), visualised by the development of intramyocardial haemorrhage (IMH) or microvascular obstruction (MVO) in cardiac MRI. Thus far, translation of novel pharmacological strategies from preclinical studies to target either IRI or HF post MI have been largely unsuccessful. Anti-inflammatory therapies also carry the risk of affecting the immune system. Fractalkine (FKN, CX3CL1) is a unique chemokine, present as a transmembrane protein on the endothelium, or following cleavage as a soluble ligand, attracting leukocyte subsets expressing the corresponding receptor CX3CR1. We have shown previously that the fractalkine receptor CX3CR1 is associated with MVO in patients undergoing primary PCI. Moreover, inhibition of CX3CR1 with an allosteric small molecule antagonist (KAND567) in the rat MI model reduces acute infarct size, inflammation, and IMH. Here we review the cellular biology of fractalkine and its receptor, along with ongoing studies that introduce CX3CR1 as a future target in coronary artery disease, specifically in patients with myocardial infarction.

CX3CL1 induces cell migration and invasion through ICAM-1 expression in oral squamous cell carcinoma cells

Human oral squamous cell carcinoma (OSCC) has been associated with a relatively low survival rate over the years and is characterized by a poor prognosis. C-X3-C motif chemokine ligand 1 (CX3CL1) has been involved in advanced migratory cells. Overexpressed CX3CL1 promotes several cellular responses related to cancer metastasis, including cell movement, migration and invasion in tumour cells. However, CX3CL1 controls the migration ability, and its molecular mechanism in OSCC remains unknown. The present study confirmed that CX3CL1 increased cell movement, migration and invasion. The CX3CL1-induced cell motility is upregulated through intercellular adhesion molecule-1 (ICAM-1) expression in OSCC cells. These effects were significantly suppressed when OSCC cells were pre-treated with CX3CR1 monoclonal antibody (mAb) and small-interfering RNA (siRNA). The CX3CL1-CX3CR1 axis activates promoted PLCβ/PKCα/c-Src phosphorylation. Furthermore, CX3CL1 enhanced activator protein-1 (AP-1) activity. The CX3CR1 mAb and PLCβ, PKCα, c-Src inhibitors reduced CX3CL1-induced c-Jun phosphorylation, c-Jun translocation into the nucleus and c-Jun binding to the ICAM-1 promoter. The present results reveal that CX3CL1 induces the migration of OSCC cells by promoting ICAM-1 expression through the CX3CR1 and the PLCβ/PKCα/c-Src signal pathway, suggesting that CX3CL1-CX3CR1-mediated signalling is correlated with tumour motility and appealed to be a precursor for prognosis in human OSCC.

Identification of cuproptosis-associated subtypes and signature genes for diagnosis and risk prediction of Ulcerative colitis based on machine learning

Background

Ulcerative colitis (UC) is a chronic and debilitating inflammatory bowel disease that impairs quality of life. Cuproptosis, a recently discovered form of cell death, has been linked to many inflammatory diseases, including UC. This study aimed to examine the biological and clinical significance of cuproptosis-related genes in UC.

Methods

Three gene expression profiles of UC were obtained from the Gene Expression Omnibus (GEO) database to form the combined dataset. Differential analysis was performed based on the combined dataset to identify differentially expressed genes, which were intersected with cuproptosis-related genes to obtain differentially expressed cuproptosis-related genes (DECRGs). Machine learning was conducted based on DECRGs to identify signature genes. The prediction model of UC was established using signature genes, and the molecular subtypes related to cuproptosis of UC were identified. Functional enrichment analysis and immune infiltration analysis were used to evaluate the biological characteristics and immune infiltration landscape of signature genes and molecular subtypes.

Results

Seven signature genes (ABCB1, AQP1, BACE1, CA3, COX5A, DAPK2, and LDHD) were identified through the machine learning algorithms, and the nomogram built from these genes had excellent predictive performance. The 298 UC samples were divided into two subtypes through consensus cluster analysis. The results of the functional enrichment analysis and immune infiltration analysis revealed significant differences in gene expression patterns, biological functions, and enrichment pathways between the cuproptosis-related molecular subtypes of UC. The immune infiltration analysis also showed that the immune cell infiltration in cluster A was significantly higher than that of cluster B, and six of the characteristic genes (excluding BACE1) had higher expression levels in subtype B than in subtype A.

Conclusions

This study identified several promising signature genes and developed a nomogram with strong predictive capabilities. The identification of distinct subtypes of UC enhances our current understanding of UC's underlying pathogenesis and provides a foundation for personalized diagnosis and treatment in the future.

Subsets of cancer cells expressing CX3CR1 are endowed with metastasis-initiating properties and resistance to chemotherapy

Metastasis-initiating cells (MICs) display stem cell-like features, cause metastatic recurrences and defy chemotherapy, which leads to patients' demise. Here we show that prostate and breast cancer patients harbor contingents of tumor cells with high expression of CX3CR1, OCT4a (POU5F1), and NANOG. Impairing CX3CR1 expression or signaling hampered the formation of tumor spheroids by cell lines from which we isolated small subsets co-expressing CX3CR1 and stemness-related markers, similarly to patients' tumors. These rare CX3CR1^High cells show transcriptomic profiles enriched in pathways that regulate pluripotency and endowed with metastasis-initiating behavior in murine models. Cancer cells lacking these features (CX3CR1^Low) were capable of re-acquiring CX3CR1-associated features over time, implying that MICs can continuously emerge from non-stem cancer cells. CX3CR1 expression also conferred resistance to docetaxel, and prolonged treatment with docetaxel selected CX3CR1^High phenotypes with de-enriched transcriptomic profiles for apoptotic pathways. These findings nominate CX3CR1 as a novel marker of stem-like tumor cells and provide conceptual ground for future development of approaches targeting CX3CR1 signaling and (re)expression as therapeutic means to prevent or contain metastasis initiation.

Tetrahydroxy stilbene glycoside regulates TGF-β/fractalkine/CX3CR1 based on network pharmacology in APP/PS1 mouse model

Alzheimer's disease (AD) is a serious, progressive neurodegenerative disease that involves irreversible neuronal death. Tetrahydroxy stilbene glycoside (TSG) is an active compound extracted from P. multiflorum, a traditional Chinese herbal medicine, but its role in neuroprotection is unclear. Herein, we aimed to validate the effects of TSG on APP/PS1 model mice and the underlying mechanism. RNA-seq was performed to identify differentially expressed genes in APP/PS1 mouse, with PCR and immunohistochemistry used for validation. Experiments were performed after bioinformatic analysis for verification. Neuronal damage was observed by H&E staining. Key proteins involved in the pathway such as CX3CR1, Iba1 and TGF-β were examined by immunohistochemical analysis. The KEGG analysis suggested that these genes might act by multiple pathways to build the pharmacological network of TSG in AD progression. These data provide the credible evidence that TSG improved neuronal damage and regulated neuroprotective mechanisms. Together, our work has detailed the whole and major genes in APP/PS1 model mouse regulated by TSG, and highlighted the anti-inflammatory function of TSG in mediating CX3CR1 and TGF-β as the TGF-β/fractalkine/CX3XR1 signaling pathway, especially in microglia. Moreover, TSG has potential value in synaptic transmission and neurotrophic action on neurodegenerative diseases. In summary, TSG is a promising candidate for preventing and treating the progression of AD.

The Clinical Implications and Molecular Mechanism of CX3CL1 Expression in Urothelial Bladder Cancer

Background

CX3CL1 is a chemokine that may play important roles in cancer immune regulation. Its mechanism in bladder cancer (BCa) is poorly understood. The objective of the current study was to evaluate the association between CX3CL1 and BCa and the related biological mechanisms.

Methods

A total of 277 patients with BCa were enrolled in the present study. The association between CX3CL1 expression and disease outcome was evaluated. In vitro and in vivo experiments were performed using the TCCSUP cell line to investigate the function of CX3CL1 in BCa.

Results

Compared with low expression, high expression of CX3CL1 was significantly associated with poorer progression-free survival (hazard ratio [HR]=2.03, 95% confidence interval [95% CI]: 1.26-3.27, P=0.006), cancer-specific survival (HR=2.16, 95% CI: 1.59-2.93, P<0.001), and overall survival (HR=1.55, 95% CI: 1.08-2.24, P=0.039). Multivariable Cox regression analysis suggested that CX3CL1 was an independent prognostic factor for BCa outcomes. In vitro and in vivo experiments indicated that high expression of CX3CL1 was significantly associated with cell proliferation (P<0.001) and invasion (P<0.001). Gene expression profiling results showed that after CX3CL1 knockdown, CDH1 was significantly upregulated, while ETS1, RAF1, and EIF4E were significantly downregulated. Pathway enrichment analysis suggested that the ERK/MAPK signaling pathway was significantly inhibited (P<0.001).

Conclusions

CX3CL1 is an independent predictor of a poor prognosis in BCa and can promote the proliferation and invasion of BCa cells.

PET Imaging Radiotracers of Chemokine Receptors

Chemokines and chemokine receptors have been recognized as critical signal components that maintain the physiological functions of various cells, particularly the immune cells. The signals of chemokines/chemokine receptors guide various leukocytes to respond to inflammatory reactions and infectious agents. Many chemokine receptors play supportive roles in the differentiation, proliferation, angiogenesis, and metastasis of diverse tumor cells. In addition, the signaling functions of a few chemokine receptors are associated with cardiac, pulmonary, and brain disorders. Over the years, numerous promising molecules ranging from small molecules to short peptides and antibodies have been developed to study the role of chemokine receptors in healthy states and diseased states. These drug-like candidates are in turn exploited as radiolabeled probes for the imaging of chemokine receptors using noninvasive in vivo imaging, such as positron emission tomography (PET). Recent advances in the development of radiotracers for various chemokine receptors, particularly of CXCR4, CCR2, and CCR5, shed new light on chemokine-related cancer and cardiovascular research and the subsequent drug development. Here, we present the recent progress in PET radiotracer development for imaging of various chemokine receptors.

Innate Immunity to Spiral Ganglion Neuron Loss: A Neuroprotective Role of Fractalkine Signaling in Injured Cochlea

Immune system dysregulation is increasingly being attributed to the development of a multitude of neurodegenerative diseases. This, in large part, is due to the delicate relationship that exists between neurons in the central nervous system (CNS) and peripheral nervous system (PNS), and the resident immune cells that aid in homeostasis and immune surveillance within a tissue. Classically, the inner ear was thought to be immune privileged due to the presence of a blood-labyrinth barrier. However, it is now well-established that both vestibular and auditory end organs in the inner ear contain a resident (local) population of macrophages which are the phagocytic cells of the innate-immune system. Upon cochlear sterile injury or infection, there is robust activation of these resident macrophages and a predominant increase in the numbers of macrophages as well as other types of leukocytes. Despite this, the source, nature, fate, and functions of these immune cells during cochlear physiology and pathology remains unclear. Migration of local macrophages and infiltration of bone-marrow-derived peripheral blood macrophages into the damaged cochlea occur through various signaling cascades, mediated by the release of specific chemical signals from damaged sensory and non-sensory cells of the cochlea. One such signaling pathway is CX₃CL1-CX₃CR1, or fractalkine (FKN) signaling, a direct line of communication between macrophages and sensory inner hair cells (IHCs) and spiral ganglion neurons (SGNs) of the cochlea. Despite the known importance of this neuron-immune axis in CNS function and pathology, until recently it was not clear whether this signaling axis played a role in macrophage chemotaxis and SGN survival following cochlear injury. In this review, we will explore the importance of innate immunity in neurodegenerative disease development, specifically focusing on the regulation of the CX₃CL1-CX₃CR1 axis, and present evidence for a role of FKN signaling in cochlear neuroprotection.

In Sickness and in Health: The Immunological Roles of the Lymphatic System

The lymphatic system plays crucial roles in immunity far beyond those of simply providing conduits for leukocytes and antigens in lymph fluid. Endothelial cells within this vasculature are distinct and highly specialized to perform roles based upon their location. Afferent lymphatic capillaries have unique intercellular junctions for efficient uptake of fluid and macromolecules, while expressing chemotactic and adhesion molecules that permit selective trafficking of specific immune cell subsets. Moreover, in response to events within peripheral tissue such as inflammation or infection, soluble factors from lymphatic endothelial cells exert “remote control” to modulate leukocyte migration across high endothelial venules from the blood to lymph nodes draining the tissue. These immune hubs are highly organized and perfectly arrayed to survey antigens from peripheral tissue while optimizing encounters between antigen-presenting cells and cognate lymphocytes. Furthermore, subsets of lymphatic endothelial cells exhibit differences in gene expression relating to specific functions and locality within the lymph node, facilitating both innate and acquired immune responses through antigen presentation, lymph node remodeling and regulation of leukocyte entry and exit. This review details the immune cell subsets in afferent and efferent lymph, and explores the mechanisms by which endothelial cells of the lymphatic system regulate such trafficking, for immune surveillance and tolerance during steady-state conditions, and in response to infection, acute and chronic inflammation, and subsequent resolution.

Role of inflammatory chemokines in hypertension

Hypertension is associated with immune cells activation and their migration into the kidney, vasculature, heart and brain. These inflammatory mechanisms are critical for blood pressure regulation and mediate target organ damage, creating unique novel targets for pharmacological modulation. In response to angiotensin II and other pro-hypertensive stimuli, the expression of several inflammatory chemokines and their receptors is increased in the target organs, mediating homing of immune cells. In this review, we summarize the contribution of key inflammatory chemokines and their receptors to increased accumulation of immune cells in target organs and effects on vascular dysfunction, remodeling, oxidative stress and fibrosis, all of which contribute to blood pressure elevation. In particular, the role of CCL2, CCL5, CXCL8, CXCL9, CXCL10, CXCL11, CXCL16, CXCL1, CX3CL1, XCL1 and their receptors in the context of hypertension is discussed. Recent studies have tested the efficacy of pharmacological or genetic targeting of chemokines and their receptors on the development of hypertension. Promising results indicate that some of these pathways may serve as future therapeutic targets to improve blood pressure control and prevent target organ consequences including kidney failure, heart failure, atherosclerosis or cognitive impairment.

Emerging Role of Fractalkine in the Treatment of Rheumatic Diseases

Rheumatoid arthritis (RA) is an autoimmune disorder that affects joints and is characterized by synovial hyperplasia and bone erosion associated with neovascularization and infiltration of proinflammatory cells. The introduction of biological disease-modifying anti-rheumatic drugs has dramatically changed the treatment of RA over the last 20 years. However, fewer than 50% of RA patients enter remission, and 10–15% are treatment refractory. There is currently no cure for RA. Fractalkine (FKN, also known as CX3CL1) is a cell membrane-bound chemokine that can be induced on activated vascular endothelial cells. FKN has dual functions as a cell adhesion molecule and a chemoattractant. FKN binds specifically to the chemokine receptor CX3CR1, which is selectively expressed on subsets of immune cells such as patrolling monocytes and killer lymphocytes. The FKN–CX3CR1 axis is thought to play important roles in the initiation of the inflammatory cascade and can contribute to exacerbation of tissue injury in inflammatory diseases. Accordingly, studies in animal models have shown that inhibition of the FKN–CX3CR1 axis not only improves rheumatic diseases but also reduces associated complications, such as pulmonary fibrosis and cardiovascular disease. Recently, a humanized anti-FKN monoclonal antibody, E6011, showed promising efficacy with a dose-dependent clinical response and favorable safety profile in a Phase 2 clinical trial in patients with RA (NCT02960438). Taken together, the preclinical and clinical results suggest that E6011 may represent a new therapeutic approach for rheumatic diseases by suppressing a major contributor to inflammation and mitigating concomitant cardiovascular and fibrotic diseases. In this review, we describe the role of the FKN–CX3CR1 axis in rheumatic diseases and the therapeutic potential of anti-FKN monoclonal antibodies to fulfill unmet clinical needs.

Chemokine-like factor 1: A promising therapeutic target in human diseases

IMPACT STATEMENT

CKLF1, a recently identified chemokine, has been reported by a number of studies to play important roles in quite many diseases. However, the potential pathways that CKLF1 may be involved are not manifested well yet. In our review, we showed the basic molecular structure and major functions of this novel chemokine, and implication in human diseases, such as tumors. To attract more attention, we summarized its signaling pathways and clearly present them in a set of figures. With the overview of the experimental trial of CKLF1-targeting medicines in animal models, we hope to provide a few important insights about CKLF1 to both medical researchers and pharmacy.

Fractalkine/CX3CL1 in Neoplastic Processes

Fractalkine/CX3C chemokine ligand 1 (CX3CL1) is a chemokine involved in the anticancer function of lymphocytes-mainly NK cells, T cells and dendritic cells. Its increased levels in tumors improve the prognosis for cancer patients, although it is also associated with a poorer prognosis in some types of cancers, such as pancreatic ductal adenocarcinoma. This work focuses on the 'hallmarks of cancer' involving CX3CL1 and its receptor CX3CR1. First, we describe signal transduction from CX3CR1 and the role of epidermal growth factor receptor (EGFR) in this process. Next, we present the role of CX3CL1 in the context of cancer, with the focus on angiogenesis, apoptosis resistance and migration and invasion of cancer cells. In particular, we discuss perineural invasion, spinal metastasis and bone metastasis of cancers such as breast cancer, pancreatic cancer and prostate cancer. We extensively discuss the importance of CX3CL1 in the interaction with different cells in the tumor niche: tumor-associated macrophages (TAM), myeloid-derived suppressor cells (MDSC) and microglia. We present the role of CX3CL1 in the development of active human cytomegalovirus (HCMV) infection in glioblastoma multiforme (GBM) brain tumors. Finally, we discuss the possible use of CX3CL1 in immunotherapy.

The Requirement for US28 During Cytomegalovirus Latency Is Independent of US27 and US29 Gene Expression

The ability to establish a latent infection with periodic reactivation events ensures herpesviruses, like human cytomegalovirus (HCMV), lifelong infection, and serial passage. The host-pathogen relationship throughout HCMV latency is complex, though both cellular and viral factors influence the equilibrium between latent and lytic infection. We and others have shown one of the viral-encoded G protein-coupled receptors, US28, is required for HCMV latency. US28 potentiates signals both constitutively and in response to ligand binding, and we previously showed deletion of the ligand binding domain or mutation of the G protein-coupling domain results in the failure to maintain latency similar to deletion of the entire US28 open reading frame (ORF). Interestingly, a recent publication detailed an altered phenotype from that previously reported, showing US28 is required for viral reactivation rather than latency, suggesting the US28 ORF deletion impacts transcription of the surrounding genes. Here, we show an independently generated US28-stop mutant, like the US28 ORF deletion mutant, fails to maintain latency in hematopoietic cells. Further, we found US27 and US29 transcription in each of these mutants was comparable to their expression during wild type infection, suggesting neither US28 mutant alters mRNA levels of the surrounding genes. Finally, infection with a US28 ORF deletion virus expressed US27 protein comparable to its expression following wild type infection. In sum, our new data strongly support previous findings from our lab and others, detailing a requirement for US28 during HCMV latent infection.

Lack of Fractalkine Receptor on Macrophages Impairs Spontaneous Recovery of Ribbon Synapses After Moderate Noise Trauma in C57BL/6 Mice

Noise trauma causes loss of synaptic connections between cochlear inner hair cells (IHCs) and the spiral ganglion neurons (SGNs). Such synaptic loss can trigger slow and progressive degeneration of SGNs. Macrophage fractalkine signaling is critical for neuron survival in the injured cochlea, but its role in cochlear synaptopathy is unknown. Fractalkine, a chemokine, is constitutively expressed by SGNs and signals via its receptor CX₃CR1 that is expressed on macrophages. The present study characterized the immune response and examined the function of fractalkine signaling in degeneration and repair of cochlear synapses following noise trauma. Adult mice wild type, heterozygous and knockout for CX₃CR1 on a C57BL/6 background were exposed for 2 h to an octave band noise at 90 dB SPL. Noise exposure caused temporary shifts in hearing thresholds without any evident loss of hair cells in CX₃CR1 heterozygous mice that have intact fractalkine signaling. Enhanced macrophage migration toward the IHC-synaptic region was observed immediately after exposure in all genotypes. Synaptic immunolabeling revealed a rapid loss of ribbon synapses throughout the basal turn of the cochlea of all genotypes. The damaged synapses spontaneously recovered in mice with intact CX₃CR1. However, CX₃CR1 knockout (KO) animals displayed enhanced synaptic degeneration that correlated with attenuated suprathreshold neural responses at higher frequencies. Exposed CX₃CR1 KO mice also exhibited increased loss of IHCs and SGN cell bodies compared to exposed heterozygous mice. These results indicate that macrophages can promote repair of damaged synapses after moderate noise trauma and that repair requires fractalkine signaling.

CX3CR1 differentiates F4/80low monocytes into pro-inflammatory F4/80high macrophages in the liver

The expression of chemokine receptor CX₃CR1 is related to migration and signaling in cells of the monocyte-macrophage lineage. The precise roles of CX₃CR1 in the liver have been investigated but not clearly elucidated. Here, we investigated the roles of CX₃CR1 in hepatic macrophages and liver injury. Hepatic and splenic CX₃CR1^lowF4/80^low monocytes and CX₃CR1^lowCD16⁻ monocytes were differentiated into CX₃CR1^highF4/80^high or CX₃CR1^highCD16⁺ macrophages by co-culture with endothelial cells. Moreover, CX₃CL1 deficiency in human umbilical vein endothelial cells (HUVECs) attenuated the expression of interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α), whereas recombinant CX₃CL1 treatment reversed this expression in co-cultured monocytes. Upon treatment with clodronate liposome, hepatic F4/80^high macrophages were successfully depleted at day 2 and recovered similarly in CX₃CR1^+/GFP and CX₃CR1^GFP/GFP mice at week 4, suggesting a CX₃CR1-independent replacement. However, F4/80^high macrophages of CX₃CR1^+/GFP showed a stronger pro-inflammatory phenotype than CX₃CR1^GFP/GFP mice. In clodronate-treated chimeric CX₃CR1^+/GFP and CX₃CR1^GFP/GFP mice, CX₃CR1⁺F4/80^high macrophages showed higher expression of IL-1β and TNF-α than CX₃CR1⁻F4/80^high macrophages. In alcoholic liver injury, despite the similar frequency of hepatic F4/80^high macrophages, CX₃CR1^GFP/GFP mice showed reduced liver injury, hepatic fat accumulation, and inflammatory responses than CX₃CR1^+/GFP mice. Thus, CX₃CR1 could be a novel therapeutic target for pro-inflammatory macrophage-mediated liver injury.

Low shear stress upregulates the expression of fractalkine through the activation of mitogen-activated protein kinases in endothelial cells

Fractalkine (FKN) is a cytokine which plays an important role in atherosclerosis and other inflammatory diseases. Studies have shown that FKN induces integrin-independent leukocyte adhesion to primary endothelial cells under physiological flow conditions. Further, increased expression of FKN has been demonstrated in atherosclerotic lesions induced by low shear stress. However, the signal transduction mechanisms involved in low shear stress-induced FKN upregulation are not well characterized. In this study, EA.hy926 cells were subjected to varying intensity of fluid shear stress for different time durations. Further, mRNA and protein expressions of FKN were assessed by quantitative real-time PCR and Western blotting, respectively. Upregulation of FKN expression, which was induced via activation of mitogen-activated protein kinases signaling pathway under conditions of low shear stress, was studied both in the presence and absence of inhibitors. Low shear stress (∼4.58 dyne/cm²) for more than 1 h promoted FKN expression and activated the extracellular signal-regulated kinase (ERK)1/2, p38, and Jun N-terminal kinase (JNK) mitogen-activated protein kinases signaling pathways by their phosphorylation. Inhibitors of ERK1/2, p38, and JNK pathways downregulated the FKN expression. In this study, fluid shear stress affected FKN expression in endothelial cells via activation of ERK1/2, p38, and JNK in a time-dependent manner. Our findings serve to advance the theoretical basis for prevention and treatment of atherosclerosis.

US28: HCMV's Swiss Army Knife

US28 is one of four G protein coupled receptors (GPCRs) encoded by human cytomegalovirus (HCMV). The US28 protein (pUS28) is a potent signaling molecule that alters a variety of cellular pathways that ultimately alter the host cell environment. This viral GPCR is expressed not only in the context of lytic replication but also during viral latency, highlighting its multifunctional properties. pUS28 is a functional GPCR, and its manipulation of multiple signaling pathways likely impacts HCMV pathogenesis. Herein, we will discuss the impact of pUS28 on both lytic and latent infection, pUS28-mediated signaling and its downstream consequences, and the influence this viral GPCR may have on disease states, including cardiovascular disease and cancer. We will also discuss the potential for and progress towards exploiting pUS28 as a novel therapeutic to combat HCMV.

Allergenic proteases cleave the chemokine CX3CL1 directly from the surface of airway epithelium and augment the effect of rhinovirus

CX3CL1 has been implicated in allergen-induced airway CD4⁺ T-lymphocyte recruitment in asthma. As epidemiological evidence supports a viral infection-allergen synergy in asthma exacerbations, we postulated that rhinovirus (RV) infection in the presence of allergen augments epithelial CX3CL1 release. Fully differentiated primary bronchial epithelial cultures were pretreated apically with house dust mite (HDM) extract and infected with rhinovirus-16 (RV16). CX3CL1 was measured by enzyme-linked immunosorbent assay and western blotting, and shedding mechanisms assessed using inhibitors, protease-activated receptor-2 (PAR-2) agonist, and recombinant CX3CL1-expressing HEK293T cells. Basolateral CX3CL1 release was unaffected by HDM but stimulated by RV16; inhibition by fluticasone or GM6001 implicated nuclear factor-κB and ADAM (A Disintegrin and Metalloproteinase) sheddases. Conversely, apical CX3CL1 shedding was stimulated by HDM and augmented by RV16. Although fluticasone or GM6001 reduced RV16+HDM-induced apical CX3CL1 release, heat inactivation or cysteine protease inhibition completely blocked CX3CL1 shedding. The HDM effect was via enzymatic cleavage of CX3CL1, not PAR-2 activation, yielding a product mitogenic for smooth muscle cells. Extracts of Alternaria fungus caused similar CX3CL1 shedding. We have identified a novel mechanism whereby allergenic proteases cleave CX3CL1 from the apical epithelial surface to yield a biologically active product. RV16 infection augmented HDM-induced CX3CL1 shedding-this may contribute to synergy between allergen exposure and RV infection in triggering asthma exacerbations and airway remodeling.

Microglia and Aging: The Role of the TREM2-DAP12 and CX3CL1-CX3CR1 Axes

Depending on the species, microglial cells represent 5-20% of glial cells in the adult brain. As the innate immune effector of the brain, microglia are involved in several functions: regulation of inflammation, synaptic connectivity, programmed cell death, wiring and circuitry formation, phagocytosis of cell debris, and synaptic pruning and sculpting of postnatal neural circuits. Moreover, microglia contribute to some neurodevelopmental disorders such as Nasu-Hakola disease (NHD), and to aged-associated neurodegenerative diseases, such as Alzheimer's disease (AD), Parkinson's disease (PD), and others. There is evidence that human and rodent microglia may become senescent. This event determines alterations in the microglia activation status, associated with a chronic inflammation phenotype and with the loss of neuroprotective functions that lead to a greater susceptibility to the neurodegenerative diseases of aging. In the central nervous system (CNS), Triggering Receptor Expressed on Myeloid Cells 2-DNAX activation protein 12 (TREM2-DAP12) is a signaling complex expressed exclusively in microglia. As a microglial surface receptor, TREM2 interacts with DAP12 to initiate signal transduction pathways that promote microglial cell activation, phagocytosis, and microglial cell survival. Defective TREM2-DAP12 functions play a central role in the pathogenesis of several diseases. The CX3CL1 (fractalkine)-CX3CR1 signaling represents the most important communication channel between neurons and microglia. The expression of CX3CL1 in neurons and of its receptor CX3CR1 in microglia determines a specific interaction, playing fundamental roles in the regulation of the maturation and function of these cells. Here, we review the role of the TREM2-DAP12 and CX3CL1-CX3CR1 axes in aged microglia and the involvement of these pathways in physiological CNS aging and in age-associated neurodegenerative diseases.

Genetic disruption of fractalkine signaling leads to enhanced loss of cochlear afferents following ototoxic or acoustic injury

Cochlear hair cells are vulnerable to a variety of insults like acoustic trauma and ototoxic drugs. Such injury can also lead to degeneration of spiral ganglion neurons (SGNs), but this occurs over a period of months to years. Neuronal survival is necessary for the proper function of cochlear prosthetics, therefore, it is of great interest to understand the mechanisms that regulate neuronal survival in deaf ears. We have recently demonstrated that selective hair cell ablation is sufficient to attract leukocytes into the spiral ganglion, and that fractalkine signaling plays a role in macrophage recruitment and in the survival of auditory neurons. Fractalkine (CX₃ CL1), a chemokine that regulates adhesion and migration of leukocytes is expressed by SGNs and signals to leukocytes via its receptor CX₃ CR1. The present study has extended the previous findings to more clinically relevant conditions of sensorineural hearing loss by examining the role of fractalkine signaling after aminoglycoside ototoxicity or acoustic trauma. Both aminoglycoside treatment and acoustic overstimulation led to the loss of hair cells as well as prolonged increase in the numbers of cochlear leukocytes. Lack of CX₃ CR1 did not affect macrophage recruitment after injury, but resulted in increased loss of SGNs and enhanced expression of the inflammatory cytokine interleukin-1β, when compared to mice with intact CX₃ CR1. These data indicate that the dysregulation of macrophage response caused by the absence of CX₃ CR1 may contribute to inflammation-mediated neuronal loss in the deafened ear, suggesting a key role for inflammation in the long-term survival of target-deprived afferent neurons.

Aptamer micelles targeting fractalkine-expressing cancer cells in vitro and in vivo

In this work we hypothesized that the chemokine fractalkine can serve as a cancer molecular target. We engineered aptamer micelles functionalized with an outer poly(ethylene glycol) (PEG) corona, and investigated the extent and efficacy of using them as a targeting tool against fractalkine-expressing colon adenocarcinoma cells. In vitro cell binding results showed that aptamer micelles bound and internalized to fractalkine-expressing cancer cells with the majority of the micelles found free in the cytoplasm. Minimal surface binding was observed by healthy cells. Even though partial PEGylation did not prevent serum adsorption, micelles were highly resistant to endonuclease and exonuclease degradation. In vivo biodistribution studies and confocal studies demonstrated that even though both aptamer and control micelles showed tumor accumulation, only the aptamer micelles internalized into fractalkine-expressing cancer cells, thus demonstrating the potential of the approach and showing that fractalkine may serve as a specific target for nanoparticle delivery to cancer cells.

Ly6Clo monocytes drive immunosuppression and confer resistance to anti-VEGFR2 cancer therapy

Current anti-VEGF therapies for colorectal cancer (CRC) provide limited survival benefit, as tumors rapidly develop resistance to these agents. Here, we have uncovered an immunosuppressive role for nonclassical Ly6Clo monocytes that mediates resistance to anti-VEGFR2 treatment. We found that the chemokine CX3CL1 was upregulated in both human and murine tumors following VEGF signaling blockade, resulting in recruitment of CX3CR1+Ly6Clo monocytes into the tumor. We also found that treatment with VEGFA reduced expression of CX3CL1 in endothelial cells in vitro. Intravital microscopy revealed that CX3CR1 is critical for Ly6Clo monocyte transmigration across the endothelium in murine CRC tumors. Moreover, Ly6Clo monocytes recruit Ly6G+ neutrophils via CXCL5 and produce IL-10, which inhibits adaptive immunity. Preventing Ly6Clo monocyte or Ly6G+ neutrophil infiltration into tumors enhanced inhibition of tumor growth with anti-VEGFR2 therapy. Furthermore, a gene therapy using a nanoparticle formulated with an siRNA against CX3CL1 reduced Ly6Clo monocyte recruitment and improved outcome of anti-VEGFR2 therapy in mouse CRCs. Our study unveils an immunosuppressive function of Ly6Clo monocytes that, to our knowledge, has yet to be reported in any context. We also reveal molecular mechanisms underlying antiangiogenic treatment resistance, suggesting potential immunomodulatory strategies to enhance the long-term clinical outcome of anti-VEGF therapies.

Evolution of the ability to modulate host chemokine networks via gene duplication in human cytomegalovirus (HCMV)

Human cytomegalovirus (HCMV) is a widespread pathogen that is particularly skillful at evading immune detection and defense mechanisms, largely due to extensive co-evolution with its host. One aspect of this co-evolution involves the acquisition of virally encoded G protein-coupled receptors (GPCRs) with homology to the chemokine receptor family. GPCRs are the largest family of cell surface proteins, found in organisms from yeast to humans, and they regulate a variety of cellular processes including development, sensory perception, and immune cell trafficking. The US27 and US28 genes are encoded by human and primate CMVs, but homologs are not found in the genomes of viruses infecting rodents or other species. Phylogenetic analysis was used to investigate the US27 and US28 genes, which are adjacent in the unique short (US) region of the HCMV genome, and their relationship to one another and to human chemokine receptor genes. The results indicate that both US27 and US28 share the same common ancestor with human chemokine receptor CX3CR1, suggesting that a single host gene was captured and a subsequent viral gene duplication event occurred. The US28 gene product (pUS28) has maintained the function of the ancestral gene and has the ability to bind and signal in response to CX3CL1/fractalkine, the natural ligand for CX3CR1. In contrast, pUS27 does not bind to any known chemokine ligand, and the sequence has diverged significantly, highlighted by the fact that pUS27 currently exhibits greater sequence similarity to human CCR1. While the evolutionary advantage of the gene duplication and neofunctionalization event remains unclear, the US27 and US28 genes are highly conserved among different HCMV strains and retained even in laboratory strains that have lost many virulence genes, suggesting that US27 and US28 have each evolved distinct, important functions during virus infection.

Downregulation of CX3CR1 ameliorates experimental colitis: evidence for CX3CL1-CX3CR1-mediated immune cell recruitment

PURPOSE

Inflammatory conditions like inflammatory bowel diseases (IBD) are characterized by increased immune cell infiltration. The chemokine ligand CX₃CL1 and its receptor CX₃CR1 have been shown to be involved in leukocyte adhesion, transendothelial recruitment, and chemotaxis. Therefore, the objective of this study was to describe CX3CL1-CX3CR1-mediated signaling in the induction of immune cell recruitment during experimental murine colitis.

METHODS

Acute colitis was induced by dextran sodium sulfate (DSS), and sepsis was induced by injection of lipopolysaccharide (LPS). Serum concentrations of CX₃CR1 and CX₃CL1 were measured by ELISA. Wild-type and CX₃CR1^-/- mice were challenged with DSS, and on day 6, intravital microscopy was performed to monitor colonic leukocyte and platelet recruitment. Intestinal inflammation was assessed by disease activity, histopathology, and neutrophil infiltration.

RESULTS

CX₃CR1 was upregulated in DSS colitis and LPS-induced sepsis. CX₃CR1^-/- mice were protected from disease severity and intestinal injury in DSS colitis, and CX₃CR1 deficiency resulted in reduced rolling of leukocytes and platelets.

CONCLUSIONS

In the present study, we provide evidence for a crucial role of CX₃CL1-CX₃CR1 in experimental colitis, in particular for intestinal leukocyte recruitment during murine colitis. Our findings suggest that CX₃CR1 blockade represents a potential therapeutic strategy for treatment of IBD.

Chemokine Receptor Signaling and the Hallmarks of Cancer

The chemokines are a family of chemotactic cytokines that mediate their activity by acting on seven-transmembrane-spanning G protein-coupled receptors. Both the ability of the chemokines and their receptors to form homo- and heterodimers and the promiscuity of the chemokine-chemokine receptor interaction endow this protein family with enormous signaling plasticity and complexity that are not fully understood at present. Chemokines were initially identified as essential regulators of homeostatic and inflammatory trafficking of innate and adaptive leucocytes from lymphoid organs to tissues. Chemokines also mediate the host response to cancer. Nevertheless, chemokine function in this response is not limited to regulating leucocyte infiltration into the tumor microenvironment. It is now known that chemokines and their receptors influence most-if not all-hallmark processes of cancer; they act on both neoplastic and untransformed cells in the tumor microenvironment, including fibroblasts, endothelial cells (blood and lymphatic), bone marrow-derived stem cells, and, obviously, infiltrating leucocytes. This review begins with an overview of chemokine and chemokine receptor structure, to better define how chemokines affect the proliferation, survival, stemness, and metastatic potential of neoplastic cells. We also examine the main mechanisms by which chemokines regulate tumor angiogenesis and immune cell infiltration, emphasizing the pro- and antitumorigenic activity of this protein superfamily in these interrelated processes.

Microglial CX3CR1 promotes adult neurogenesis by inhibiting Sirt 1/p65 signaling independent of CX3CL1

Homo and heterozygote cx3cr1 mutant mice, which harbor a green fluorescent protein (EGFP) in their cx3cr1 loci, represent a widely used animal model to study microglia and peripheral myeloid cells. Here we report that microglia in the dentate gyrus (DG) of cx3cr1^−/− mice displayed elevated microglial sirtuin 1 (SIRT1) expression levels and nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kB) p65 activation, despite unaltered morphology when compared to cx3cr1^+/− or cx3cr1^+/+ controls. This phenotype was restricted to the DG and accompanied by reduced adult neurogenesis in cx3cr1^−/− mice. Remarkably, adult neurogenesis was not affected by the lack of the CX₃CR1-ligand, fractalkine (CX₃CL1). Mechanistically, pharmacological activation of SIRT1 improved adult neurogenesis in the DG together with an enhanced performance of cx3cr1^−/− mice in a hippocampus-dependent learning and memory task. The reverse condition was induced when SIRT1 was inhibited in cx3cr1^−/− mice, causing reduced adult neurogenesis and lowered hippocampal cognitive abilities. In conclusion, our data indicate that deletion of CX₃CR1 from microglia under resting conditions modifies brain areas with elevated cellular turnover independent of CX₃CL1.

Fractalkine shedding is mediated by p38 and the ADAM10 protease under pro-inflammatory conditions in human astrocytes

Background

The fractalkine (CX3CR1) ligand is expressed in astrocytes and reported to be neuroprotective. When cleaved from the membrane, soluble fractalkine (sCX3CL1) activates the receptor CX3CR1. Although somewhat controversial, CX3CR1 is reported to be expressed in neurons and microglia. The membrane-bound form of CX3CL1 additionally acts as an adhesion molecule for microglia and infiltrating white blood cells. Much research has been done on the role of fractalkine in neuronal cells; however, little is known about the regulation of the CX3CL1 ligand in astrocytes.

Methods

The mechanisms involved in the up-regulation and cleavage of CX3CL1 from human astrocytes were investigated using immunocytochemistry, Q-PCR and ELISA. All statistical analysis was performed using GraphPad Prism 5.

Results

A combination of ADAM17 (TACE) and ADAM10 protease inhibitors was found to attenuate IL-1β-, TNF-α- and IFN-γ-induced sCX3CL1 levels in astrocytes. A specific ADAM10 (but not ADAM17) inhibitor also attenuated these effects, suggesting ADAM10 proteases induce release of sCX3CL1 from stimulated human astrocytes. A p38 MAPK inhibitor also attenuated the levels of sCX3CL1 upon treatment with IL-1β, TNF-α or IFN-γ. In addition, an IKKβ inhibitor significantly reduced the levels of sCX3CL1 induced by IL-1β or TNF-α in a concentration-dependent manner, suggesting a role for the NF-kB pathway.

Conclusions

In conclusion, this study shows that the release of soluble astrocytic fractalkine is regulated by ADAM10 proteases with p38 MAPK also playing a role in the fractalkine shedding event. These findings are important for understanding the role of CX3CL1 in healthy and stimulated astrocytes and may benefit our understanding of this pathway in neuro-inflammatory and neurodegenerative diseases.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-016-0659-7) contains supplementary material, which is available to authorized users.

Mutually Supportive Mechanisms of Inflammation and Vascular Remodeling

Chronic inflammation is often accompanied by angiogenesis, the development of new blood vessels from existing ones. This vascular response is a response to chronic hypoxia and/or ischemia, but is also contributory to the progression of disorders including atherosclerosis, arthritis, and tumor growth. Proinflammatory and proangiogenic mediators and signaling pathways form a complex and interrelated network in these conditions, and many factors exert multiple effects. Inflammation drives angiogenesis by direct and indirect mechanisms, promoting endothelial proliferation, migration, and vessel sprouting, but also by mediating extracellular matrix remodeling and release of sequestered growth factors, and recruitment of proangiogenic leukocyte subsets. The role of inflammation in promoting angiogenesis is well documented, but by facilitating greater infiltration of leukocytes and plasma proteins into inflamed tissues, angiogenesis can also propagate chronic inflammation. This review examines the mutually supportive relationship between angiogenesis and inflammation, and considers how these interactions might be exploited to promote resolution of chronic inflammatory or angiogenic disorders.

NFAT isoforms play distinct roles in TNFα-induced retinal leukostasis

The objective of this study was to determine the role of individual NFAT isoforms in TNFα-induced retinal leukostasis. To this end, human retinal microvascular endothelial cells (HRMEC) transfected with siRNA targeting individual NFAT isoforms were treated with TNFα, and qRT-PCR was used to examine the contribution of each isoform to the TNFα-induced upregulation of leukocyte adhesion proteins. This showed that NFATc1 siRNA increased ICAM1 expression, NFATc2 siRNA reduced CX3CL1, VCAM1, SELE, and ICAM1 expression, NFATc3 siRNA increased CX3CL1 and SELE expression, and NFATc4 siRNA reduced SELE expression. Transfected HRMEC monolayers were also treated with TNFα and assayed using a parallel plate flow chamber, and both NFATc2 and NFATc4 knockdown reduced TNFα-induced cell adhesion. The effect of isoform-specific knockdown on TNFα-induced cytokine production was also measured using protein ELISAs and conditioned cell culture medium, and showed that NFATc4 siRNA reduced CXCL10, CXCL11, and MCP-1 protein levels. Lastly, the CN/NFAT-signaling inhibitor INCA-6 was shown to reduce TNFα-induced retinal leukostasis in vivo. Together, these studies show a clear role for NFAT-signaling in TNFα-induced retinal leukostasis, and identify NFATc2 and NFATc4 as potentially valuable therapeutic targets for treating retinopathies in which TNFα plays a pathogenic role.

Microglia in Health and Disease

Microglia, the major myeloid cells of the central nervous system (CNS) are implicated in physiologic processes and in the pathogenesis of several CNS disorders. Since their initial description early in the 20th century, our ability to identify and isolate microglia has significantly improved and new research is providing insight into the functions of these cells in sickness and in health. Here, we review recent advances in our understanding of the role of microglia in physiological and pathological processes of the CNS with a focus on multiple sclerosis and Alzheimer's disease. Because of the prominent roles CX3CR1 and its ligand fractalkine played in bringing about these advances, we discuss the physiological and pathological roles of microglia as viewed from the CX3CR1-fractalkine perspective, providing a unique viewpoint. Based on the most recent studies of molecular profiling of microglia, we also propose a molecular and functional definition of microglia that incorporates the properties attributed to these cells in recent years.

Inflammatory reaction after traumatic brain injury: therapeutic potential of targeting cell-cell communication by chemokines

Traumatic brain injury (TBI) affects millions of people worldwide every year. The primary impact initiates the secretion of pro- and anti-inflammatory factors, subsequent recruitment of peripheral immune cells, and activation of brain-resident microglia and astrocytes. Chemokines are major mediators of peripheral blood cell recruitment to damaged tissue, including the TBI brain. Here we review the involvement of specific chemokine pathways in TBI pathology and attempts to modulate these pathways for therapeutic purposes. We focus on chemokine (C-C motif) ligand 2/chemokine (C-C motif) receptor 2 (CCL2/CCR2) and chemokine (C-X-C motif) ligand 12/chemokine (C-X-C motif) receptor 4 (CXCL12/CXCR4). Recent microarray and multiplex expression profiling have also implicated CXCL10 and CCL5 in TBI pathology. Chemokine (C-X3-C motif) ligand 1/chemokine (C-X3-C motif) receptor 1 (CX3CL1/CX3CR1) signaling in the context of TBI is also discussed. Current literature suggests that modulating chemokine signaling, especially CCL2/CCR2, may be beneficial in TBI treatment.

Inhibition of the activation and recruitment of microglia-like cells protects against neomycin-induced ototoxicity

One of the most unfortunate side effects of aminoglycoside (AG) antibiotics such as neomycin is that they target sensory hair cells (HCs) and can cause permanent hearing impairment. We have observed HC loss and microglia-like cell (MLC) activation in the inner ear (cochlea) following neomycin administration. We focused on CX3CL1, a membrane-bound glycoprotein expressed on neurons and endothelial cells, as a way to understand how the MLCs are activated and the role these cells play in HC loss. CX3CL1 is the exclusive ligand for CX3CR1, which is a chemokine receptor expressed on the surface of macrophages and MLCs. In vitro experiments showed that the expression levels of CX3CL1 and CX3CR1 increased in the cochlea upon neomycin treatment, and CX3CL1 was expressed on HCs, while CX3CR1 was expressed on MLCs. When cultured with 1 μg/mL exogenous CX3CL1, MLCs were activated by CX3CL1, and the cytokine level was increased in the cochleae leading to apoptosis in the HCs. In CX3CR1 knockout mice, a significantly greater number of cochlear HCs survived than in wild-type mice when the cochlear explants were cultured with neomycin in vitro. Furthermore, inhibiting the activation of MLCs with minocycline reduced the neomycin-induced HC loss and improved the hearing function in neomycin-treated mice in vivo. Our results demonstrate that CX3CL1-induced MLC activation plays an important role in the induction of HC death and provide evidence for CX3CL1 and CX3CR1 as promising new therapeutic targets for the prevention of hearing loss.

The role of fractalkine (CX3CL1) in regulation of CD4(+) cell migration to the central nervous system in patients with relapsing-remitting multiple sclerosis

Fractalkine (CX3CL1) levels are increased in the cerebrospinal fluid (CSF) of patients with clinically isolated syndrome (CIS), as well as in the CSF and serum samples from patients with relapsing-remitting multiple sclerosis (RRMS). A higher percentage of circulating CD4(+) T-cells expressed its surface receptor (CX3CR1) and intracellular adhesion molecule (ICAM-1) in RRMS patients in comparison to healthy controls (HCs). The CX3CR1(+)ICAM-1(+)CD4(+) T-cells are enriched in the CSF of the RRMS patients. In vitro migration studies revealed that CD4(+) T-cells, which migrated toward a CX3CL1 gradient, expressed higher levels of ICAM-1 than non-migrating cells. CX3CL1 significantly increased IFN-γ and TNF-α gene expression and IFN-γ secretion by CD4(+) T-cells derived from the RRMS patients. CX3CL1 upregulated ICAM-1 expression on the surface of RRMS patient-derived but not HC-derived CD4(+) T-cells. Thus, CX3CL1 induces recruitment of CX3CR1(+)ICAM-1(+)CD4(+) T-cells into the central nervous system (CNS) during the early inflammatory response in MS.

The evolving biology of microglia in Alzheimer's disease

Alzheimer's disease (AD) is typified by a robust microglial-mediated inflammatory response within the brain. Indeed, microglial accumulation around plaques in AD is one of the classical hallmarks of the disease pathology. Although microglia have the capacity to remove β-amyloid deposits and alleviate disease pathology, they fail to do so. Instead, they become chronically activated and promote inflammation-mediated impairment of cognition and cytotoxicity. However, if microglial function could be altered to engage their phagocytic response, promote their tissue maintenance functions, and prevent release of factors that promote tissue damage, this could provide therapeutic benefit. This review is focused on the current knowledge of microglial homeostatic mechanisms in AD, and mechanisms involved in the regulation of microglial phenotype in this context.

Fractalkine receptor chemokine (CX3CR1) influences on cervical and lumbar disc herniation

BACKGROUND

Herniation of nuclear or disc material along with, inflammatory chemokines such as prostaglandin E2, interleukin-6, matrix metalloproteinase and nitric oxide has definite correlation, possibly they are over produced. CX3CL1 and its receptor (CX3CR1) are part of chemokine system involved in leukocyte recruitment and adhesion in chronic inflammatory disease, but its role in spinal herniated nucleus pulposus (HNP) is unknown. We evaluated the expression of CX3CL1 and CX3CR1 in patients with disc herniation to clarify the role of CX3CL1 and CX3CR1 in the disc degeneration and to compare between cervical and lumbar HNP.

MATERIALS AND METHODS

The mRNA concentrations of CX3CL1/CX3CR1 chemokine were analyzed in the surgically obtained disc specimens from C-HNP (n = 13) and L-HNP (n = 13) by real-time polymerase chain reaction (PCR). The localization of CX3CL1/CX3CR1 chemokine in the disc of C-HNP and L-HNP patients was determined using immunohistochemical study. Blood samples from patients with C-HNP and L-HNP patients were stained for CX3CR1 with flow cytometric analysis.

RESULTS

The CX3CL1 positive cell ratio in the discs was observed in both groups by immunohistochemical study. CX3CR1 was strongly expressed on endothelial cells in C-spine disc, but sparely expressed in L-spine disc. There was greater CX3CR1 mRNA expression in C-HNP patients than in L-HNP patients as quantified by reversal transcription-PCR (P = 0.010). CX3CR1 positive cell frequencies and CX3CR1 expression levels were increased in CD4 (+) T-cells and natural killer (NK) cells from patients with C-HNP (P = 0.210 and P = 0.040).

CONCLUSIONS

This study identified that increases in CX3CL1 and CX3CR1-expressing cells are significantly related to pathomechanism of HNP for the first time. Especially, CD4 (+) T-cells and NK cells expressing CX3CR1 may play an important role in developing C-HNP.

CX3CL1, a chemokine finely tuned to adhesion: critical roles of the stalk glycosylation and the membrane domain

The multi-domain CX3CL1 transmembrane chemokine triggers leukocyte adherence without rolling and migration by presenting its chemokine domain (CD) to its receptor CX3CR1. Through the combination of functional adhesion assays with structural analysis using FRAP, we investigated the functional role of the other domains of CX3CL1, i.e., its mucin stalk, transmembrane domain, and cytosolic domain. Our results indicate that the CX3CL1 molecular structure is finely adapted to capture CX3CR1 in circulating cells and that each domain has a specific purpose: the mucin stalk is stiffened by its high glycosylation to present the CD away from the membrane, the transmembrane domain generates the permanent aggregation of an adequate amount of monomers to guarantee adhesion and prevent rolling, and the cytosolic domain ensures adhesive robustness by interacting with the cytoskeleton. We propose a model in which quasi-immobile CX3CL1 bundles are organized to quickly generate adhesive patches with sufficiently high strength to capture CX3CR1+ leukocytes but with sufficiently low strength to allow their patrolling behavior.

Fractalkine (CX3CL1), a new factor protecting β-cells against TNFα

Objective

We have previously shown the existence of a muscle–pancreas intercommunication axis in which CX3CL1 (fractalkine), a CX3C chemokine produced by skeletal muscle cells, could be implicated. It has recently been shown that the fractalkine system modulates murine β-cell function. However, the impact of CX3CL1 on human islet cells especially regarding a protective role against cytokine-induced apoptosis remains to be investigated.

Methods

Gene expression was determined using RNA sequencing in human islets, sorted β- and non-β-cells. Glucose-stimulated insulin secretion (GSIS) and glucagon secretion from human islets was measured following 24 h exposure to 1–50 ng/ml CX3CL1. GSIS and specific protein phosphorylation were measured in rat sorted β-cells exposed to CX3CL1 for 48 h alone or in the presence of TNFα (20 ng/ml). Rat and human β-cell apoptosis (TUNEL) and rat β-cell proliferation (BrdU incorporation) were assessed after 24 h treatment with increasing concentrations of CX3CL1.

Results

Both CX3CL1 and its receptor CX3CR1 are expressed in human islets. However, CX3CL1 is more expressed in non-β-cells than in β-cells while its receptor is more expressed in β-cells. CX3CL1 decreased human (but not rat) β-cell apoptosis. CX3CL1 inhibited human islet glucagon secretion stimulated by low glucose but did not impact human islet and rat sorted β-cell GSIS. However, CX3CL1 completely prevented the adverse effect of TNFα on GSIS and on molecular mechanisms involved in insulin granule trafficking by restoring the phosphorylation (Akt, AS160, paxillin) and expression (IRS2, ICAM-1, Sorcin, PCSK1) of key proteins involved in these processes.

Conclusions

We demonstrate for the first time that human islets express and secrete CX3CL1 and CX3CL1 impacts them by decreasing glucagon secretion without affecting insulin secretion. Moreover, CX3CL1 decreases basal apoptosis of human β-cells. We further demonstrate that CX3CL1 protects β-cells from the adverse effects of TNFα on their function by restoring the expression and phosphorylation of key proteins of the insulin secretion pathway.

Chemokines in the balance: maintenance of homeostasis and protection at CNS barriers

In the adult central nervous system (CNS), chemokines and their receptors are involved in developmental, physiological and pathological processes. Although most lines of investigation focus on their ability to induce the migration of cells, recent studies indicate that chemokines also promote cellular interactions and activate signaling pathways that maintain CNS homeostatic functions. Many homeostatic chemokines are expressed on the vasculature of the blood brain barrier (BBB) including CXCL12, CCL19, CCL20, and CCL21. While endothelial cell expression of these chemokines is known to regulate the entry of leukocytes into the CNS during immunosurveillance, new data indicate that CXCL12 is also involved in diverse cellular activities including adult neurogenesis and neuronal survival, having an opposing role to the homeostatic chemokine, CXCL14, which appears to regulate synaptic inputs to neural precursors. Neuronal expression of CX₃CL1, yet another homeostatic chemokine that promotes neuronal survival and communication with microglia, is partly regulated by CXCL12. Regulation of CXCL12 is unique in that it may regulate its own expression levels via binding to its scavenger receptor CXCR7/ACKR3. In this review, we explore the diverse roles of these and other homeostatic chemokines expressed within the CNS, including the possible implications of their dysfunction as a cause of neurologic disease.

Lymphatic Regulation of Cellular Trafficking

Lymphatic vessels play vital roles in immune surveillance and immune regulation by conveying antigen loaded dendritic cells, memory T cells, macrophages and neutrophils from the peripheral tissues to draining lymph nodes where they initiate as well as modify immune responses. Until relatively recently however, there was little understanding of how entry and migration through lymphatic vessels is organized or the specific molecular mechanisms that might be involved. Within the last decade, the situation has been transformed by an explosion of knowledge generated largely through the application of microscopic imaging, transgenic animals, specific markers and function blocking mAbs that is beginning to provide a rational conceptual framework. This article provides a critical review of the recent literature, highlighting seminal discoveries that have revealed the fascinating ultrastructure of leucocyte entry sites in lymphatic vessels, as well as generating controversies over the involvement of integrin adhesion, chemotactic and haptotactic mechanisms in DC entry under normal and inflamed conditions. It also discusses the major changes in lymphatic architecture that occur during inflammation and the different modes of leucocyte entry and trafficking within inflamed lymphatic vessels, as well as presenting a timely update on the likely role of hyaluronan and the major lymphatic endothelial hyaluronan receptor LYVE-1 in leucocyte transit.

Neuron-glia crosstalk in health and disease: fractalkine and CX3CR1 take centre stage

An essential aspect of normal brain function is the bidirectional interaction and communication between neurons and neighbouring glial cells. To this end, the brain has evolved ligand-receptor partnerships that facilitate crosstalk between different cell types. The chemokine, fractalkine (FKN), is expressed on neuronal cells, and its receptor, CX(3)CR1, is predominantly expressed on microglia. This review focuses on several important functional roles for FKN/CX(3)CR1 in both health and disease of the central nervous system. It has been posited that FKN is involved in microglial infiltration of the brain during development. Microglia, in turn, are implicated in the developmental synaptic pruning that occurs during brain maturation. The abundance of FKN on mature hippocampal neurons suggests a homeostatic non-inflammatory role in mechanisms of learning and memory. There is substantial evidence describing a role for FKN in hippocampal synaptic plasticity. FKN, on the one hand, appears to prevent excess microglial activation in the absence of injury while promoting activation of microglia and astrocytes during inflammatory episodes. Thus, FKN appears to be neuroprotective in some settings, whereas it contributes to neuronal damage in others. Many progressive neuroinflammatory disorders that are associated with increased microglial activation, such as Alzheimer's disease, show disruption of the FKN/CX(3)CR1 communication system. Thus, targeting CX(3)CR1 receptor hyperactivation with specific antagonists in such neuroinflammatory conditions may eventually lead to novel neurotherapeutics.

Hypertrophy of the ligament flavum in degenerative lumbar stenosis associated with the increased expression of fractalkine (CX3CL1)/CX3CR1 chemokine

Fractalkine (CX3CL1) and its receptor (CX3CR1) comprise a chemokine system involved in leukocyte recruitment and adhesion in chronic inflammatory disease, but its role in spinal diseases is unknown. The purpose of this study is to investigate the role of CX3CL1/CX3CR1 chemokine on hypertrophy of the ligamentum flavum (LF) in degenerative lumbar stenosis (DLS) compared with that of non-degenerative spinal condition (NDS) of the lumbar spine and correlation between expression of CX3CL1/CX3CR1 chemokine and thickness of LF. The mRNA concentrations of CX3CL1/CX3CR1 chemokine were analyzed in the surgically obtained LF specimens from DLS (n = 10) and NDS (n = 11) by real-time PCR. The localization of CX3CL1/CX3CR1 chemokine within the LF was determined using immunohistochemical study. Plasma levels of soluble FKN (sFKN) were measured by enzyme-linked immunosorbent assay, respectively. The thickness of the LF was measured with axial T1-weighted MRI. The cells that express CX3CL1/CX3CR1 chemokine ratio in the LF observed in DLS group were substantially higher than in NDS group. In ELISA, the plasma levels of sFKN was significantly increased in DLS compared with patients in the other groups (p = 0.006). There was greater CX3CL1/CX3CR1 expression in DLS as quantified by RT-PCR (p = 0.004, 0.010). Thickness of LF in patients was significantly correlated with serum CX3CL1 level (R(2) = 0.824, p = 0.003) and with mRNA expression of CX3CL1/CX3CR1 (R(2) = 0.671, p = 0.000) (R(2) = 0.514, p = 0.001). This study identified for the first time that increases in CX3CL1 and CX3CR1-expressing cells are significantly related to LF hypertrophy.

The chemokine CX3CL1 promotes trafficking of dendritic cells through inflamed lymphatics

Tissue inflammation is characterised by increased trafficking of antigen-loaded dendritic cells (DCs) from the periphery via afferent lymphatics to draining lymph nodes, with a resulting stimulation of ongoing immune responses. Transmigration across lymphatic endothelium constitutes the first step in this process and is known to involve the chemokine CCL21 and its receptor CCR7. However, the precise details of DC transit remain obscure and it is likely that additional chemokine-receptor pairs have roles in lymphatic vessel entry. Here, we report that the transmembrane chemokine CX3CL1 (fractalkine) is induced in inflamed lymphatic endothelium, both in vitro in TNF-α-treated human dermal lymphatic endothelial cells (HDLECs) and in vivo in a mouse model of skin hypersensitivity. However, unlike blood endothelial cells, which express predominantly transmembrane CX3CL1 as a leukocyte adhesion molecule, HDLECs shed virtually all CX3CL1 at their basolateral surface through matrix metalloproteinases. We show for the first time that both recombinant soluble CX3CL1 and endogenous secreted CX3CL1 promote basolateral-to-luminal migration of DCs across HDLEC monolayers in vitro. Furthermore, we show in vivo that neutralising antibodies against CX3CL1 dramatically reduce allergen-induced trafficking of cutaneous DCs to draining lymph nodes as assessed by FITC skin painting in mice. Finally, we show that deletion of the CX3CL1 receptor in Cx3cr1^−/− DCs results in markedly delayed lymphatic trafficking in vivo and impaired translymphatic migration in vitro, thus establishing a previously unrecognised role for this atypical chemokine in regulating DC trafficking through the lymphatics.

A novel CXCL10-based GPI-anchored fusion protein as adjuvant in NK-based tumor therapy

Background

Cellular therapy is a promising therapeutic strategy for malignant diseases. The efficacy of this therapy can be limited by poor infiltration of the tumor by immune effector cells. In particular, NK cell infiltration is often reduced relative to T cells. A novel class of fusion proteins was designed to enhance the recruitment of specific leukocyte subsets based on their expression of a given chemokine receptor. The proteins are composed of an N-terminal chemokine head, the mucin domain taken from the membrane-anchored chemokine CX3CL1, and a C-terminal glycosylphosphatidylinositol (GPI) membrane anchor replacing the normal transmembrane domain allowing integration of the proteins into cell membranes when injected into a solid tumor. The mucin domain in conjunction with the chemokine head acts to specifically recruit leukocytes expressing the corresponding chemokine receptor.

Methodology/Principal Findings

A fusion protein comprising a CXCL10 chemokine head (CXCL10-mucin-GPI) was used for proof of concept for this approach and expressed constitutively in Chinese Hamster Ovary cells. FPLC was used to purify proteins. The recombinant proteins efficiently integrated into cell membranes in a process dependent upon the GPI anchor and were able to activate the CXCR3 receptor on lymphocytes. Endothelial cells incubated with CXCL10-mucin-GPI efficiently recruited NK cells in vitro under conditions of physiologic flow, which was shown to be dependent on the presence of the mucin domain. Experiments conducted in vivo using established tumors in mice suggested a positive effect of CXCL10-mucin-GPI on the recruitment of NK cells.

Conclusions

The results suggest enhanced recruitment of NK cells by CXCL10-mucin-GPI. This class of fusion proteins represents a novel adjuvant in cellular immunotherapy. The underlying concept of a chemokine head fused to the mucin domain and a GPI anchor signal sequence may be expanded into a broader family of reagents that will allow targeted recruitment of cells in various settings.

Regulation of adaptive immunity by the fractalkine receptor during autoimmune inflammation

Fractalkine, a chemokine anchored to neurons or peripheral endothelial cells, serves as an adhesion molecule or as a soluble chemoattractant. Fractalkine binds CX3CR1 on microglia and circulating monocytes, dendritic cells, and NK cells. The aim of this study is to determine the role of CX3CR1 in the trafficking and function of myeloid cells to the CNS during experimental autoimmune encephalomyelitis (EAE). Our results show that, in models of active EAE, Cx3cr1(-/-) mice exhibited more severe neurologic deficiencies. Bone marrow chimeric mice confirmed that CX3CR1 deficiency in bone marrow enhanced EAE severity. Notably, CX3CR1 deficiency was associated with an increased accumulation of CD115(+)Ly6C(-)CD11c(+) dendritic cells into EAE-affected brains that correlated with enhanced demyelination and neuronal damage. Furthermore, higher IFN-γ and IL-17 levels were detected in cerebellar and spinal cord tissues of CX3CR1-deficient mice. Analyses of peripheral responses during disease initiation revealed a higher frequency of IFN-γ- and IL-17-producing T cells in lymphoid tissues of CX3CR1-deficient as well as enhanced T cell proliferation induced by CX3CR1-deficient dendritic cells. In addition, adoptive transfer of myelin oligodendrocyte glycoprotein35-55-reactive wild-type T cells induced substantially more severe EAE in CX3CR1-deficient recipients when compared with wild-type recipients. Collectively, the data demonstrate that besides its role in chemoattraction, CX3CR1 is a key regulator of myeloid cell activation contributing to the establishment of adaptive immune responses.