CX3CL1 deficiency ameliorates inflammation, apoptosis and accelerates osteogenic differentiation, mineralization in LPS-treated MC3T3-E1 cells via its receptor CX3CR1

BACKGROUND

Osteoporosis is a devastating skeletal disease responsible for bone fragility and fracture. CX3C chemokine ligand 1 (CX3CL1) is an inflammatory chemokine which has been identified to possess increased expression in the serum of postmenopausal osteoporotic patients. This paper was to illuminate the impacts of CX3CL1 on inflammation, apoptosis and osteogenic differentiation, mineralization in LPS-treated osteoblasts and investigate the regulatory mechanism.

METHODS

The viability of MC3T3-E1 cells exposed to elevating doses of LPS was detected by CCK-8 assay. CX3CL1 and C-X3-C motif chemokine receptor 1 (CX3CR1) expression were detected by RT-qPCR and western blot. CX3CR1 expression was examined again following CX3CL1 depletion. The binding of CX3CL1 with CX3CR1 was testified through Co-IP assay. In MC3T3-E1 cells co-transduced with CX3CL1 interference and CX3CR1 overexpression plasmids following LPS exposure, cell activity and inflammation were separately estimated via CCK-8 assay and RT-qPCR. Apoptosis was measured by TUNEL assay and western blot. Osteoblast differentiation was evaluated by ALP activity assay, RT-qPCR and western blot. Osteoblast mineralization was assessed by ARS staining, RT-qPCR and western blot. Results The experimental data presented that LPS attenuated the viability and enhanced CX3CL1 and CX3CR1 expression in MC3T3-E1 cells in a dose-dependent manner. CX3CR1 interacted with CX3CL1 and was positively modulated by CX3CL1. The suppressive role of CX3CL1 absence in LPS-evoked viability decrease, inflammation and apoptosis in MC3T3-E1 cells was reversed by CX3CR1 elevation. Besides, CX3CR1 reversed the promoted osteoblast differentiation and mineralization imposed by CX3CL1 interference.

CONCLUSIONS

CX3CL1 knockdown eased inflammation, apoptosis and promoted osteogenic differentiation, mineralization in MC3T3-E1 cells upon LPS exposure through down-regulating CX3CR1.

Opposite effects of CX3CR1 receptor polymorphisms V249I and T280M on the development of acute coronary syndrome

Several lines of evidence suggest that the chemokine fractalkine (FKN) and its receptor CX3CR1 contribute to the accumulation of leukocytes in the atherosclerotic plaque. The M280 allele of the CX3CR1T280M polymorphism modulates leukocyte recruitment and is associated with lower prevalence of cardiovascular disease. The influence of V249I, another CX3CR1 poly-morphism, is discussed controversially. We investigated the association of the alleles M280 and I249 of CX3CR1 with coronary artery disease (CAD) and with acute coronary syndrome (ACS). Additionally, we assessed their association with the soluble ligand FKN and inflammatory activation measured by high-sensitivity C-reactive protein (hsCRP). The genotypes of the V249I and T280M polymorphisms were determined in 1152 patients with suspected CAD. 720 (62.5%) individuals showed significant CAD with an ACS prevalence of 59.3%. Using multivariate regression, we found a harmful influence of I249 (adjusted OR=1.8, P<0.03) and a protective effect of M280 (adjusted OR=0.6, P<0.04) on the occurrence of ACS in patients with CAD. Correspondingly, patients with I249 but without M280 (17%) were at elevated risk of ACS (OR=1.6, P<0.04). During ACS these patients (carrying only I249) had significantly higher circulating concentrations of FKN and high sensitivity C-reactive protein (1.9– and 1.6-fold). We found no association of the I249 or the M280 allele with the occurrence of CAD. In conclusion, I249 and M280 have opposite effects on the occurrence of ACS. The presence of I249 not“balanced” by M280 confers an elevated risk of ACS. A FKN-mediated enhanced inflammatory activation might explain this increased risk.

Interleukin-15 inhibits smooth muscle cell proliferation and hyaluronan production in rat ductus arteriosus

Neointimal cushion formation (NCF) is an important vascular remodeling for anatomical closure of the ductus arteriosus (DA). Inflammatory responses to vascular injury or atherosclerosis are known to be associated with the pathogenesis of NCF. We found that the expression of interleukin (IL)-15 mRNA was significantly higher in rat DA than in the aorta. IL-15 immunoreactivity was detected predominantly in the internal elastic laminae (IEL) and to a lesser extent in smooth muscle cells (SMCs) in rat DA. Prostaglandin E (PGE) increased the expression of IL-15 mRNA in cultured DA SMCs. IL-15 significantly attenuated the platelet-derived growth factor (PDGF)-BB-mediated SMC proliferation, but did not change SMC migration. IL-15 significantly attenuated PGE1-induced hyaluronic acid (HA) production in a dose-dependent manner, which is a potent stimulator of NCF. Accordingly, IL-15 might have an inhibitory effect on the physiologic vascular remodeling processes in closing the DA.

Fractalkine is a novel chemoattractant for rheumatoid arthritis fibroblast-like synoviocyte signaling through MAP kinases and Akt

OBJECTIVE

Fibroblast-like synoviocytes (FLS) are a major constituent of the hyperplastic synovial pannus that aggressively invades cartilage and bone during the course of rheumatoid arthritis (RA). Fractalkine (FKN/CX(3)CL1) expression is up-regulated in RA synovium and RA synovial fluid. While RA FLS express the FKN receptor, CX(3)CR1, the pathophysiologic relevance of FKN stimulation of RA FLS is not understood. This study was undertaken to better characterize the relationship between FKN and the RA FLS that both produce it and express its receptor.

METHODS

RA FLS were subjected to chemotaxis and proliferation assays, Western blotting, enzyme-linked immunosorbent assays, and filamentous actin staining to characterize the relationship between FKN and RA FLS.

RESULTS

FKN secretion by RA FLS was regulated mainly by tumor necrosis factor alpha. Stimulation of RA FLS with FKN led to significant cytoskeletal rearrangement but no proliferation. Chemotaxis assays revealed that FKN was a novel chemoattractant for RA FLS. Stimulation of RA FLS with FKN resulted in activation of MAP kinases and Akt. JNK, ERK-1/2, and Akt (at both Ser-473 and Thr-308) were each up-regulated in a time-dependent manner. Inhibition of ERK-1/2-mediated signaling, but not JNK or Akt, significantly repressed FKN-induced RA FLS migration.

CONCLUSION

These findings indicate a novel role of FKN in regulating RA FLS cytoskeletal structure and migration. FKN specifically induces RA FLS phosphorylation of the MAP kinases JNK and ERK-1/2, as well as full activation of Akt.

The chemokine system in arteriogenesis and hind limb ischemia

Chemokines (chemotactic cytokines) are important in the recruitment of leukocytes to injured tissues and, as such, play a pivotal role in arteriogenesis and the tissue response to ischemia. Hind limb ischemia represents a complex model with arteriogenesis (collateral artery formation) occurring in tissues with normal perfusion while areas exhibiting ischemic necrosis undergo angiogenesis and skeletal muscle regeneration; monocytes and macrophages play an important role in all three of these processes. In addition to leukocyte trafficking, chemokines are produced by and chemokine receptors are present on diverse cell types, including myoblasts, endothelial, and smooth muscle cells. Thus, the chemokine system may have direct effects as well as inflammatory-mediated effects on arteriogenesis, angiogenesis, and skeletal muscle regeneration. This article reviews the complexity of the hind limb ischemia model and the role of the chemokine system in arteriogenesis and the tissue response to ischemia. Special emphasis will be placed on the roles of monocytes/macrophages and CCL2/monocyte chemotactic protein-1 (MCP-1) in these processes.

Chemokine Fractalkine Mediates Leukocyte Recruitment to Inflammatory Endothelial Cells in Flowing Whole Blood

Background— The membrane-bound chemokine fractalkine (CX 3 CL1) is expressed on various cell types such as activated endothelial cells and has been implicated in the inflammatory process of atherosclerosis. The aim of the present study was to dissect the role of fractalkine in leukocyte recruitment to inflamed endothelium under arterial shear forces.

Methods and Results— With the use of immunofluorescence and laminar flow assays, the present study shows that human umbilical vein endothelial cells stimulated with tumor necrosis factor-α and interferon-γ abundantly express CX 3 CL1 and promote substantial leukocyte accumulation under arterial flow conditions. In the presence of high shear, firm adhesion of leukocytes to inflamed endothelial cells is reduced by ≈40% by a function-blocking anti-fractalkine antibody or by an antibody directed against the fractalkine receptor (CX 3 CR1). With the use of intravital video-fluorescence microscopy we demonstrate that inhibition of fractalkine signaling attenuates leukocyte adhesion to the atherosclerotic carotid artery of apolipoprotein E–deficient mice, which suggests that the CX 3 CL1-CX 3 CR1 axis is critically involved in leukocyte adhesion to inflamed endothelial cells under high shear forces both in vitro and in vivo. Surprisingly, platelets were strictly required for fractalkine-induced leukocyte adhesion at high shear rates. Correspondingly, specific inhibition of platelet adhesion to inflamed endothelial cells also significantly reduced leukocyte accumulation. We show that both soluble and membrane-bound fractalkine induces platelet degranulation and subsequent surface expression of P-selectin, which thereby promotes direct platelet-leukocyte interaction.

Conclusion— Fractalkine expressed by inflamed endothelial cells triggers P-selectin exposure on adherent platelets, which thereby initiates the local accumulation of leukocytes under arterial shear, an essential step in the development of atherosclerotic lesions.

Regulated shedding of transmembrane chemokines by the disintegrin and metalloproteinase 10 facilitates detachment of adherent leukocytes

CX3CL1 (fractalkine) and CXCL16 are unique members of the chemokine family because they occur not only as soluble, but also as membrane-bound molecules. Expressed as type I transmembrane proteins, the ectodomain of both chemokines can be proteolytically cleaved from the cell surface, a process known as shedding. Our previous studies showed that the disintegrin and metalloproteinase 10 (ADAM10) mediates the largest proportion of constitutive CX3CL1 and CXCL16 shedding, but is not involved in the phorbolester-induced release of the soluble chemokines (inducible shedding). In this study, we introduce the calcium-ionophore ionomycin as a novel, very rapid, and efficient inducer of CX3CL1 and CXCL16 shedding. By transfection in COS-7 cells and ADAM10-deficient murine embryonic fibroblasts combined with the use of selective metalloproteinase inhibitors, we demonstrate that the inducible generation of soluble forms of these chemokines is dependent on ADAM10 activity. Analysis of the C-terminal cleavage fragments remaining in the cell membrane reveals multiple cleavage sites used by ADAM10, one of which is preferentially used upon stimulation with ionomycin. In adhesion studies with CX3CL1-expressing ECV-304 cells and cytokine-stimulated endothelial cells, we demonstrate that induced CX3CL1 shedding leads to the release of bound monocytic cell lines and PBMC from their cellular substrate. These data provide evidence for an inducible release mechanism via ADAM10 potentially important for leukocyte diapedesis.

HYPOXIA-INDUCED ASTROCYTES PROMOTE THE MIGRATION OF NEURAL PROGENITOR CELLS VIA VASCULAR ENDOTHELIAL FACTOR, STEM CELL FACTOR, STROMAL-DERIVED FACTOR-1? AND MONOCYTE CHEMOATTRACTANT PROTEIN-1 UPREGULATION IN VITRO

1. The aim of the present study was to examine if and how rat hypoxia-induced astrocytes affect the migration of neural progenitor cells (NPC) and to investigate the expression patterns of some chemokines, such as vascular endothelial growth factor (VEGF), stem cell factor (SCF), stromal-derived factor-1alpha (SDF-1alpha), fractalkine and monocyte chemoattractant protein-1 (MCP-1) in hypoxia-induced astrocytes and their contribution to NPC migration in vitro. 2. Costar Transwell inserts were used for the chemotaxis assay and quantified changes in the chemokines mRNA for between 0 h and 24 h posthypoxia were tested using real-time quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) analysis. The results showed that the chemotaxis of astrocyte cells exposed to hypoxia for 18 h reached a peak value, whereas the chemotaxis of astrocytes exposed to hypoxia for 24 h began to decrease compared with those exposed to hypoxia for 18 h. Hypoxia upregulated chemokine VEGF, SCF, SDF-1alpha and MCP-1 expression in a time-dependent manner but downregulated fractalkine expression in astrocytes. In addition, the time points of the peak expressions for VEGF, SCF, SDF-1alpha and MCP-1 were similar to the time point of maximum NPC migration. 3. Specific inhibitors that block the binding of specific chemokines to its receptors were used for analysing the contribution of the chemokine to NPC migration. When VEGF, SCF, SDF-1alpha and MCP-1 were each inhibited independently, NPC migration was reduced. When they were inhibited together, NPC migration was obviously inhibited compared with both the control and single-block cultures, which implies that the migratory effect of hypoxia-induced astrocytes was synergetic by several chemokines. 4. In conclusion, we demonstrated the time-dependent manner of NPC migration promotion by hypoxia-induced astrocytes. We also provide evidence that soluble factors, such as VEGF, SCF, SDF-1alpha and MCP-1, released from astrocytes, direct the migration of NPC under hypoxic circumstances. Given that astrocytes were activated to all hypoxia-ischaemia diseases, these results indicate an important role for astrocytes in directing NPC replacement therapy in the central nervous system.

Role of the CX3CR1/p38 MAPK pathway in spinal microglia for the development of neuropathic pain following nerve injury-induced cleavage of fractalkine

Accumulating evidence suggests that microglial cells in the spinal cord play an important role in the development of neuropathic pain. However, it remains largely unknown how glia interact with neurons in the spinal cord after peripheral nerve injury. Recent studies suggest that the chemokine fractalkine may mediate neural/microglial interaction via its sole receptor CX3CR1. We have examined how fractalkine activates microglia in a neuropathic pain condition produced by spinal nerve ligation (SNL). SNL induced an upregulation of CX3CR1 in spinal microglia that began on day 1, peaked on day 3, and maintained on day 10. Intrathecal injection of a neutralizing antibody against CX3CR1 suppressed not only mechanical allodynia but also the activation of p38 MAPK in spinal microglia following SNL. Conversely, intrathecal infusion of fractalkine produced a marked p38 activation and mechanical allodynia. SNL also induced a dramatic reduction of the membrane-bound fractalkine in the dorsal root ganglion, suggesting a cleavage and release of this chemokine after nerve injury. Finally, application of fractalkine to spinal slices did not produce acute facilitation of excitatory synaptic transmission in lamina II dorsal horn neurons, arguing against a direct action of fractalkine on spinal neurons. Collectively, our data suggest that (a) fractalkine cleavage (release) after nerve injury may play an important role in neural-glial interaction, and (b) microglial CX3CR1/p38 MAPK pathway is critical for the development of neuropathic pain.

Exclusive increase of CX3CR1+CD28-CD4+ T cells in inflammatory bowel disease and their recruitment as intraepithelial lymphocytes

BACKGROUND

CX3CL1/Fractalkine (FKN) has been reported to play important roles in various inflammatory diseases. We examined the role of FKN and its receptor CX3CR1 in T-cell migration in the inflammatory bowel diseases (IBDs), ulcerative colitis (UC) and Crohn's disease (CD).

METHODS

CX3CR1 expression on peripheral CD4(+) cells from normal controls (NL n = 24) and IBD patients (UC n = 28, CD n = 26) was examined using flow cytometry. CX3CR1(+)CD4(+) T cells were further characterized for surface antigens, cytokine production, and cytotoxic granule release by flow cytometry and ELISA. FKN expression in 53 colonic biopsy specimens (UC n = 20, CD n = 23, NL n = 10) was analyzed by quantitative PCR and immunohistochemistry. Isolated lamina propria and intraepithelial lymphocytes were also analyzed by flow cytometry (UC n = 10, CD n = 10, NL n = 6).

RESULTS

CX3CR1(+)CD4(+) cells were increased in IBD while they were virtually absent in controls. Upregulation of CX3CR1 on CD4(+) T cells was positively correlated with disease activity. These unique T cells expressed markers for both effector memory and cytotoxic cells. Interestingly, CX3CR1 was expressed on CD4(+) T cells lacking CD28. CX3CR1(+)CD28(-)CD4(+) cells produced more IFN-gamma and TNF-alpha than CX3CR1(-) counterparts and released cytotoxic granules. FKN mRNA was upregulated in inflamed colonic tissues and robust expression of FKN was immunohistochemically observed on epithelial cells. Although CX3CR1(+) CD4(+) cells could not be detected in the gut, CD28(-)CD4(+) cells were found in IBD mainly as intraepithelial lymphocytes.

CONCLUSIONS

FKN/CX3CR1 may contribute to the pathogenesis of IBD through the emergence of unique CX3CR1(+)CD28(-)CD4(+) T cells that can act both as proinflammatory and cytotoxic cells.

Elevated levels of fractalkine expression and accumulation of CD16+ monocytes in glomeruli of active lupus nephritis

BACKGROUND

Fractalkine (Fkn) is a chemokine that affects cells expressing its receptor, CX3CR1, including CD16-positive (CD16+) monocytes/macrophages (CD16+ Mos). The relationship of levels of glomerular Fkn expression and infiltration by CD16+ Mos with the severity and diversity of glomerular lesions in human lupus nephritis is not known.

STUDY DESIGN

Retrospective cross-sectional analysis of variables observed in biopsy specimens.

SETTINGS & PARTICIPANTS

88 patients with systemic lupus erythematosus.

PREDICTOR

Histological class and severity of lupus nephritis according to the International Society of Nephrology/Renal Pathology Society and clinicopathologic factors.

OUTCOMES

Outcome variables are assays related to the degree of glomerular Fkn expression and CD16+ Mo infiltration.

MEASUREMENTS

Immunohistological grading of Fkn staining, number of CD16+ Mos, and messenger RNA levels of Fkn and CD16 in glomeruli.

RESULTS

Patients with proliferative lupus nephritis (class III and IV glomeruli) showed significantly greater glomerular Fkn expression and CD16+ Mo counts than those with other classes. Infiltrating CD16+ Mos within glomeruli expressed CX3CR1. Moreover, glomerular Fkn expression significantly correlated with the histopathologic activity index and CD16+ Mo counts, and CD16+ Mo counts significantly correlated with serum levels of blood urea nitrogen, complement (CH50), and anti-DNA antibody; estimated glomerular filtration rate; and urinary protein excretion. Glucocorticoid therapy had a tendency to decrease both glomerular Fkn expression and CD16+ Mo counts.

LIMITATIONS

Only frozen biopsy specimens (from 49 patients) were analyzed for the evaluation of glomerular Fkn expression.

CONCLUSION

Disease activity and proliferative glomerular lupus nephritis lesions are associated with the glomerular Fkn expression and CD16+ Mo accumulation.

NK cells stimulate recruitment of CXCR3+ T cells to the brain during Plasmodium berghei-mediated cerebral malaria

NK cells are cytotoxic lymphocytes that also secrete regulatory cytokines and can therefore influence adaptive immune responses. NK cell function is largely controlled by genes present in a genomic region named the NK complex. It has been shown that the NK complex is a genetic determinant of murine cerebral malaria pathogenesis mediated by Plasmodium berghei ANKA. In this study, we show that NK cells are required for cerebral malaria disease induction and the control of parasitemia. NK cells were found infiltrating brains of cerebral malaria-affected mice. NK cell depletion resulted in inhibition of T cell recruitment to the brain of P. berghei-infected animals. NK cell-depleted mice displayed down-regulation of CXCR3 expression and a significant reduction of T cells migrating in response to IFN-gamma-inducible protein 10, indicating that this chemokine pathway plays an essential role in leukocyte trafficking leading to cerebral disease and fatalities.

Atherogenic Lipids Induce Adhesion of Human Coronary Artery Smooth Muscle Cells to Macrophages by Up-regulating Chemokine CX3CL1 on Smooth Muscle Cells in a TNFα-NFκB-dependent Manner

Recent genetic evidence has implicated the adhesive chemokine CX3CL1 and its leukocyte receptor CX3CR1 in atherosclerosis. We previously proposed a mechanism involving foam cell anchorage to vascular smooth muscle cells because: 1) CX3CL1 and CX3CR1 are expressed by both cell types in mouse and human atherosclerotic lesions; 2) foam cells are reduced in lesions in cx3cr1(-/-)apoE(-/-) mice; and 3) proatherogenic lipids (oxidized low density lipoprotein [oxLDL] and oxidized linoleic acid derivatives) induce adhesion of primary human macrophages to primary human coronary artery smooth muscle cells (CASMCs) in vitro in a macrophage CX3CR1-dependent manner. Here we analyze this concept further by testing whether atherogenic lipids regulate expression and function of CX3CL1 and CX3CR1 on CASMCs. We found that both oxLDL and oxidized linoleic acid derivatives indirectly up-regulated CASMC CX3CL1 at both the protein and mRNA levels through an autocrine feedback loop involving tumor necrosis factor alpha production and NF-kappaB signaling. Oxidized lipids also up-regulated CASMC CX3CR1 but through a different mechanism. Oxidized lipid stimulation also increased adhesion of macrophages to CASMCs when CASMCs were stimulated prior to assay, and a synergistic pro-adhesive effect was observed when both cell types were prestimulated. Selective inhibition with a CX3CL1-specific blocking antibody indicated that adhesion was strongly CASMC CX3CL1-dependent. These findings support the hypothesis that CX3CR1 and CX3CL1 mediate heterotypic anchorage of foam cells to CASMCs in the context of atherosclerosis and suggest that this chemokine/chemokine receptor pair may be considered as a pro-inflammatory target for therapeutic intervention in atherosclerotic cardiovascular disease.

Surface-exposed adeno-associated virus Vp1-NLS capsid fusion protein rescues infectivity of noninfectious wild-type Vp2/Vp3 and Vp3-only capsids but not that of fivefold pore mutant virions

Over the past 2 decades, significant effort has been dedicated to the development of adeno-associated virus (AAV) as a vector for human gene therapy. However, understanding of the virus with respect to the functional domains of the capsid remains incomplete. In this study, the goal was to further examine the role of the unique Vp1 N terminus, the N terminus plus the recently identified nuclear localization signal (NLS) (J. C. Grieger, S. Snowdy, and R. J. Samulski, J. Virol 80:5199-5210, 2006), and the virion pore at the fivefold axis in infection. We generated two Vp1 fusion proteins (Vp1 and Vp1NLS) linked to the 8-kDa chemokine domain of rat fractalkine (FKN) for the purpose of surface exposure upon assembly of the virion, as previously described (K. H. Warrington, Jr., O. S. Gorbatyuk, J. K. Harrison, S. R. Opie, S. Zolotukhin, and N. Muzyczka, J. Virol 78:6595-6609, 2004). The unique Vp1 N termini were found to be exposed on the surfaces of these capsids and maintained their phospholipase A2 (PLA2) activity, as determined by native dot blot Western and PLA2 assays, respectively. Incorporation of the fusions into AAV type 2 capsids lacking a wild-type Vp1, i.e., Vp2/Vp3 and Vp3 capsid only, increased infectivity by 3- to 5-fold (Vp1FKN) and 10- to 100-fold (Vp1NLSFKN), respectively. However, the surface-exposed fusions did not restore infectivity to AAV virions containing mutations at a conserved leucine (Leu336Ala, Leu336Cys, or Leu336Trp) located at the base of the fivefold pore. EM analyses suggest that Leu336 may play a role in global structural changes to the virion directly impacting downstream conformational changes essential for infectivity and not only have local effects within the pore, as previously suggested.

Recruitment of CD16+ monocytes into synovial tissues is mediated by fractalkine and CX3CR1 in rheumatoid arthritis patients

CD16+ monocytes, identified as a minor population of monocytes in human peripheral blood, have been implicated in several inflammatory diseases, including rheumatoid arthritis (RA). Fractalkine (FKN, CX3CL1), a member of the CX3 C subfamily, is induced by pro-inflammatory cytokines, while a receptor for FKN, CX3CR1, is capable of mediating both leukocyte migration and firm adhesion. Here, we investigated the role of FKN and CX3CR1 in activation of CD16+ monocytes and their recruitment into synovial tissues in RA patients. High levels of soluble FKN were detected in the synovial fluid and sera of RA patients. Circulating CD16+ monocytes showed a higher level of CX3CR1 expression than CD16- monocytes in both RA patients and healthy subjects. High level expression of CX3CR1 was also seen in CD16+ monocytes localized to the lining layer in RA synovial tissue. In the in vitro culture experiments, IL-10 induced CX3CR1 expression on the surface of monocytes, and TNFalpha induced membrane-bound FKN as well as soluble FKN expression in synovial fibroblasts. Moreover, soluble FKN was capable of inducing IL-1beta and IL-6 by activated monocytes. These results suggest that FKN might preferentially mediate migration and recruitment of CD16+ monocytes, and might contribute to synovial tissue inflammation.

Sequential processing of the transmembrane chemokines CX3CL1 and CXCL16 by alpha- and gamma-secretases

The chemokines CX3CL1/Fractalkine and CXCL16 are expressed as transmembrane molecules and can mediate cell-cell-adhesion. By proteolytic processing, CX3CL1 and CXCL16 are released from the cell surface by proteolytic shedding resulting in the generation of soluble chemoattractants. This ectodomain release is mediated by the alpha-secretase-like activity of the two disintegrins and metalloproteinases ADAM10 and ADAM17. Using CX3CL1 and CXCL16 constructs C-terminally fused to two Z-domains of Protein A (2Z-tag) we detect C-terminal fragments (CTFs) of both chemokines resulting from ADAM10-mediated cleavages at multiple sites as examined by inhibitor studies. Furthermore, inhibitor studies as well as genetic studies using presenilin 1/2-deficient cell lines suggest the involvement of gamma-secretase-but not beta-secretase-like activity in the processing of transmembrane chemokines. The combination of alpha- and gamma-secretase and proteasomal inhibitors points towards a sequential processing of transmembrane chemokines by first ADAM10 and then gamma-secretases and possible further degradation. This proteolytic processing cascade of transmembrane chemokines is similar to that described for Notch and E-cadherin where CTFs generated by gamma-secretase serve as intracellular signal transmitters.

Fractalkine (CX3CL1)- and interleukin-2-enriched neuroblastoma microenvironment induces eradication of metastases mediated by T cells and natural killer cells

Fractalkine (FKN) is a unique CX3C chemokine (CX3CL1) known to induce both adhesion and migration of leukocytes mediated by a membrane-bound and a soluble form, respectively. Its function is mediated through CX3C receptor (CX3CR), which is expressed by T(H)1 immune cells including T cells and natural killer (NK) cells. FKN was shown to be expressed in >90% of 68 neuroblastoma samples as determined by cDNA microarray analysis. Here, we characterized the effect of FKN in the neuroblastoma microenvironment using a syngeneic model genetically engineered to secrete FKN. We show FKN-mediated migration, adhesion, and IFN-gamma secretion of immune effector cells, but limited antineuroblastoma activity, in vitro and in vivo. Therefore, we tested the hypothesis that a combined increase of FKN and interleukin-2 (IL-2) in the neuroblastoma microenvironment induces an effective antitumor immune response. For this purpose, IL-2 was targeted to ganglioside GD2, which is highly expressed on neuroblastoma tissue, using an anti-GD2 antibody IL-2 immunocytokine (ch14.18-IL-2). Only mice bearing FKN- and IL-2-enriched neuroblastoma tumors exhibited a reduction in primary tumor growth and a complete eradication of experimental liver metastases. The depletion of T cells and NK cells in vivo abrogated the effect, and these effector cells showed the highest cytolytic activity in vitro. Finally, only the FKN- and IL-2-enriched neuroblastoma microenvironment resulted in T-cell activation and the release of proinflammatory cytokines. In summary, we showed for the first time the immunologic mechanisms by which targeted IL-2 treatment of neuroblastoma with an FKN-rich microenvironment induces an effective antitumor response.

Targeted delivery of CX3CL1 to multiple lung tumors by mesenchymal stem cells

MSCs are nonhematopoietic stem cells capable of differentiating into various mesoderm-type cells. MSCs have been considered to be a potential vehicle for cell-based gene therapy because MSCs are relatively easily expanded in vitro and have the propensity to migrate to and proliferate in the tumor tissue after systemic administration. Here, we demonstrated the tropism of mouse MSCs to tumor cells in vitro and multiple tumor tissues in the lung after i.v. injection of green fluorescent protein-positive MSCs in vivo. We transduced CX3CL1 (fractalkine), an immunostimulatory chemokine, to the mouse MSCs ex vivo using an adenoviral vector with the Arg-Gly-Asp-4C peptide in the fiber knob. Intravenous injection of CX3CL1-expressing MSCs to the mice bearing lung metastases of C26 and B16F10 cells strongly inhibited the development of lung metastases and thus prolonged the survival of these tumor-bearing mice. This antitumor effect depended on both innate and adaptive immunity. These results suggest that MSCs can be used as a vehicle for introducing biological agents into multiple lung tumor tissues. Disclosure of potential conflicts of interest is found at the end of this article.

Fractalkine-CX3CR1 axis regulates tumor cell cycle and deteriorates prognosis after radical resection for hepatocellular carcinoma

BACKGROUND AND OBJECTIVES

Fractalkine is the only CX3C chemokine, and its receptor, CX3CR1, is expressed on NK cells, CD8+ T cells, monocytes, and dendritic cells (DC). Although studies have reported that fractalkine regulates the host immune response, the roles of the fractalkine-CX3CR1 axis in tumor biology and the clinical results of hepatocellular carcinoma (HCC) remain unknown.

METHODS

Fractalkine and CX3CR1 expression in HCC were evaluated and compared with the clinicopathologic features, including tumor progression determined by proliferating cell nuclear antigen (PCNA) antibody and patient prognosis after surgery.

RESULTS

Tumors with high expression of both fractalkine and CX3CR1 had significantly fewer intra- and extrahepatic recurrences, a low PCNA labeling index (PCNALI), and different histological grades. Patients with tumors that expressed both had a significantly better prognosis in terms of disease-free (DFS) and overall survival (OAS), and this finding was identified as one of the independent prognostic factors in the multivariate analysis.

CONCLUSIONS

Our results suggest that the fractalkine-CX3CR1 axis plays a pivotal role in the prognosis of patients with HCC, which might arise from the known modulation of the host immune response, and that of the cell cycle in HCC.

Tissue-specific differential antitumour effect of molecular forms of fractalkine in a mouse model of metastatic colon cancer

BACKGROUND AND AIMS

Fractalkine, a chemokine that presents as both a secreted and a membrane-anchored form, has been described as having tumour-suppressive activities in standard subcutaneous models. Here, we investigate the antitumour effect of fractalkine, in its three molecular forms, in two orthotopic models of metastatic colon cancer (liver and lung) and in the standard subcutaneous model.

METHODS

We have developed models of skin tumours, liver and pulmonary metastasis and compared the extent of tumour development between C26 colon cancer cells expressing either the native, the soluble, the membrane-bound fractalkine or none.

RESULTS

The native fractalkine exhibits the strongest antitumour effect, reducing the tumour size by 93% in the skin and by 99% in the orthotopic models (p<0.0001). Its overall effect results from a critical balance between the activity of the secreted and the membrane-bound forms, balance that is itself dependent on the target tissue. In the skin, both molecular variants reduce tumour development by 66% (p<0.01). In contrast, the liver and lung metastases are only significantly reduced by the soluble form (by 96%, p<0.002) whereas the membrane-bound variant exerts a barely significant effect in the liver (p = 0.049) and promotes tumour growth in the lungs. Moreover, we show a significant difference in the contribution of the infiltrating leukocytes to the tumour-suppressive activity of fractalkine between the standard and the orthotopic models.

CONCLUSIONS

Fractalkine expression by C26 tumour cells drastically reduces their metastatic potential in the two physiological target organs. Both molecular forms contribute to its antitumour potential but exhibit differential effects on tumour development depending on the target tissue.

Fractalkine expression in the rhesus monkey brain during lentivirus infection and its control by 6-chloro-2',3'-dideoxyguanosine

Existing data concerning the role of the delta-chemokine fractalkine (CX3CL1) and its receptor (CX3CR1) in lentivirus-induced encephalitis are limited and controversial. We explored, by quantitative in situ hybridization and immunohistochemistry, the cell-specific changes of CX3CL1 and CX3CR1 in rhesus macaque brain during simian immunodeficiency virus (SIV) infection and antiretroviral treatment. Neuronal expression of CX3CL1 was significantly reduced in cortex and striatum of AIDS-diseased monkeys as compared with uninfected and asymptomatic SIV-infected monkeys. CX3CL1 mRNA was increased in some endothelial cells and newly induced in astrocytes and macrophages focally in areas of SIV burden and inflammatory infiltrates. In most CX3CL1-positive astrocytes and macrophages, the transcription factor NF-kappaB was translocated to the nucleus. CX3CR1 was upregulated in scattered, nodule, and giant cell-forming microglia/macrophages and mononuclear infiltrates close to CX3CL1-induced cells in the brain. Treatment of AIDS monkeys with the central nervous system-permeant 6-chloro-2',3'-dideoxyguanosine fully reversed SIV burden, productive inflammation, nuclear NF-kappaB translocation as well as focal induction of CX3CL1 in astrocytes and macrophages and downregulation in neurons. In contrast, diffuse CX3CR1-positive microgliosis and GFAP-positive astrogliosis were partially reversed by 6-chloro-2',3'-dideoxyguanosine. Thus, focally induced CX3CL1 may be a target for therapeutic intervention to limit ongoing inflammatory infiltration into brain in lentivirus infection.

Enhanced recruitment of CX3CR1+ T cells by mucosal endothelial cell-derived fractalkine in inflammatory bowel disease

BACKGROUND & AIMS

Fractalkine (FKN/CX3CL1) is a unique chemokine combining adhesive and chemotactic properties. We investigated FKN production by the mucosal microvasculature in inflammatory bowel disease (IBD), its capacity for leukocyte recruitment into the gut, and the number of CX3CR1+ cells in the circulation and mucosa of IBD patients.

METHODS

The expression of FKN by human intestinal microvascular endothelial cells (HIMECs) and CX3CR1 by circulating cells was evaluated by flow cytometry, and mucosal CX3CR1+ cells were enumerated by immunohistochemistry. The capacity of FKN to mediate leukocyte binding to HIMECs was assessed by immunoblockade, and to induce HIMEC transmigration by a Transwell system.

RESULTS

The spontaneously low HIMEC FKN expression was enhanced markedly by tumor necrosis factor-alpha plus interferon-gamma stimulation, or direct leukocyte contact. This effect was significantly stronger in IBD than control HIMECs. Up-regulation of HIMEC FKN expression was dependent on p38 and extracellular signal-regulated kinase phosphorylation, as was abrogated by selective mitogen-activated protein kinase inhibitors. Circulating T cells contained significantly higher numbers of CX3CR1+ cells in active IBD than inactive IBD or healthy subjects, and IBD mucosa contained significantly more CX3CR1+ cells than control mucosa. Antibody-blocking experiments showed that FKN was a major contributor to T- and monocytic-cell adhesion to HIMECs. Finally, FKN enhanced the expression of active beta1 integrin on leukocytes and mediated leukocyte HIMEC transmigration.

CONCLUSIONS

In view of the capacity of FKN to mediate leukocyte adhesion, chemoattraction, and transmigration, its increased production by mucosal microvascular cells and increased numbers of circulating and mucosal CX3CR1+ cells in IBD point to a significant role of FKN in disease pathogenesis.

Fractalkine-induced smooth muscle cell proliferation in pulmonary hypertension

Pulmonary hypertension is characterised by a progressive increase in pulmonary arterial resistance due to endothelial and smooth muscle cell proliferation resulting in chronic obstruction of small pulmonary arteries. There is evidence that inflammatory mechanisms may contribute to the pathogenesis of human and experimental pulmonary hypertension. The aim of the study was to address the role of fractalkine (CX3CL1) in the inflammatory responses and pulmonary vascular remodelling of a monocrotaline-induced pulmonary hypertension model. The expression of CX3CL1 and its receptor CX3CR1 was studied in monocrotaline-induced pulmonary hypertension by means of immunohistochemistry and quantitative reverse-transcription PCR on laser-captured microdissected pulmonary arteries. It was demonstrated that CX3CL1 was expressed by inflammatory cells surrounding pulmonary arterial lesions and that smooth muscle cells from these vessels had increased CX3CR1 expression. It was then shown that cultured rat pulmonary artery smooth muscle cells expressed CX3CR1 and that CX3CL1 induced proliferation but not migration of these cells. In conclusion, the current authors proposed that fractalkine may act as a growth factor for pulmonary artery smooth muscle cells. Chemokines may thus play a role in pulmonary artery remodelling.

Expression and targeting of CX3CL1 (fractalkine) in renal tubular epithelial cells

The chemokine CX3CL1 plays a key role in glomerulonephritis and can act as both chemoattractant and adhesion molecule. CX3CL1 also is upregulated in tubulointerstitial injury, but little is known about the subcellular distribution and function of CX3CL1 in renal tubular epithelial cells (RTEC). Unexpectedly, it was found that CX3CL1 is expressed predominantly on the apical surface of tubular epithelium in human renal transplant biopsy specimens with acute rejection or acute tubular necrosis. For studying the targeting of CX3CL1 in polarized RTEC, MDCK cells that expressed untagged or green fluorescent protein-tagged CX3CL1 were generated. The chemokine was present on the apical membrane and in subapical vesicles. Apical targeting of CX3CL1 was not due to signals that were conferred by its intracellular domain, to associations with lipid rafts, or to O-glycosylation but, rather, depended on N-linked glycosylation of the protein. With the use of fluorescence recovery after photobleaching, it was found that CX3CL1 is immobile in the apical membrane. However, CX3CL1 partitioned with the triton-soluble rather than -insoluble cellular fraction, indicating that it is not associated directly with the actin cytoskeleton or with lipid rafts. Accordingly, disruption of rafts through cholesterol depletion did not render CX3CL1 mobile. For exploration of potential functions of apical CX3CL1, binding of CX3CR1-expressing leukocytes to polarized RTEC was examined. Leukocyte adhesion to the luminal surface was enhanced significantly when CX3CL1 was present. These data demonstrate that CX3CL1 is expressed preferentially on the apical membrane of RTEC and suggest a novel function for the chemokine in recruitment and retention of leukocytes in tubulointerstitial inflammation.

Involvement of the fractalkine pathway in the pathogenesis of childhood hemolytic uremic syndrome

Thrombotic microangiopathy and acute renal failure are cardinal features of postdiarrheal hemolytic uremic syndrome (HUS). These conditions are related to endothelial and epithelial cell damage induced by Shiga toxin (Stx) through the interaction with its globotriaosyl ceramide receptor. However, inflammatory processes contribute to the pathogenesis of HUS by sensitizing cells to Stx fractalkine (FKN), a CX(3)C transmembrane chemokine expressed on epithelial and endothelial cells upon activation, is involved in the selective migration and adhesion of specific leukocyte subsets to tissues. Here, we demonstrated a selective depletion of circulating mononuclear leukocytes expressing the receptor for FKN (CX(3)CR1) in patients with HUS. We found a unique phenotype in children with HUS distinct from that seen in healthy, uremic, or infected controls, in which monocytes lost CX(3)CR1, down-modulated CD62L, and increased CD16. In addition, the CD56(dim) natural killer (NK) subpopulation was decreased, leading to an altered peripheral CD56(dim)/CD56(bright) ratio from 10.0 to 4.5. It is noteworthy that a negative correlation existed between the percentage of circulating CX(3)CR1(+) leukocytes and the severity of renal failure. Finally, CX(3)CR1(+) leukocytes were observed in renal biopsies from patients with HUS. We suggest that the interaction of CX(3)CR1(+) cells with FKN present on activated endothelial cells may contribute to renal injury in HUS.

The Chemokine CX3CL1 Reduces Migration and Increases Adhesion of Neurons with Mechanisms Dependent on the β1 Integrin Subunit

Fractalkine/CX3CL1 and its specific receptor CX3CR1 are constitutively expressed in several regions of the CNS and are reported to mediate neuron-microglial interaction, synaptic transmission, and neuronal protection from toxic insults. CX3CL1 is released both by neuronal and astrocytic cells, whereas CX3CR1 is mainly expressed by microglial cells and neurons. Microglial cells efficiently migrate in response to CX3CL1, whereas no evidence is reported to date on CX3CL1-induced neuronal migration. For this reason, we have investigated in vitro the effects of CX3CL1 on basal migration of neurons and of the microglial and astrocytic populations, all these cells being obtained from the hippocampus and the cerebellum of newborn rats. We report that CX3CL1 stimulates microglial cell migration but efficiently reduces basal neuronal movement, regardless of the brain source. The effect of CX3CL1 is pertussis toxin (PTX) sensitive and PI3K dependent on hippocampal neurons, while it is PTX sensitive, PI3K dependent, and ERK dependent on cerebellar granules. Interestingly, CX3CL1 also increases neuron adhesion to the extracellular matrix component laminin, with mechanisms dependent on PTX-sensitive G proteins, and on the ERK and PI3K pathways. Both the reduction of migration and the increase of neuron adhesion require the activation of the beta(1) and alpha(6) integrin subunits with the exception of cerebellar neuron migration, which is only dependent on the beta(1) subunit. More importantly, in neurons, CX3CL1/CXCL12 cotreatment abolished the effect mediated by a single chemokine on chemotaxis and adhesion. In conclusion, our findings indicate that CX3CL1 reduces neuronal migration by increasing cell adhesion through integrin-dependent mechanisms in hippocampal and cerebellar neurons.

Chemokine fractalkine/CX3CL1 negatively modulates active glutamatergic synapses in rat hippocampal neurons

We examined the effects of the chemokine fractalkine (CX3CL1) on EPSCs evoked by electrical stimulation of Schaffer collaterals in patch-clamped CA1 pyramidal neurons from rat hippocampal slices. Acute application of CX3CL1 caused a sustained reduction of EPSC amplitude, with partial recovery after washout. CX3CL1-induced EPSC depression is postsynaptic in nature, because paired-pulse ratio was maintained, amplitude distribution of spontaneous excitatory postsynaptic currents shifted to lower values, and whole-cell current responses to AMPA were reversibly inhibited. EPSC depression by CX3CL1 is mediated by CX3CL1 receptor (CX3CR1), because CX3CL1 was unable to influence EPSC amplitude in CA1 pyramidal neurons from CX3CR1 knock-out mice. CX3CL1-induced depression of both EPSC and AMPA current was not observed in the absence of afferent fiber stimulation or AMPA receptor activation, respectively, indicating the requirement of sustained receptor activity for its development. Findings obtained from hippocampal slices, cultured hippocampal neurons, and transfected human embryonic kidney cells indicate that a Ca2+-, cAMP-, and phosphatase-dependent process is likely to modulate CX3CL1 effects because of the following: (1) CX3CL1-induced depression was antagonized by intracellular BAPTA, 8Br-cAMP, phosphatase inhibitors, and pertussis toxin (PTX); (2) CX3CL1 inhibited forskolin-induced cAMP formation sensitive to PTX; and (3) CX3CL1 inhibited forskolin-induced Ser845 GluR1 phosphorylation, which was sensitive to PTX and dependent on Ca2+ and phosphatase activity. Together, these findings indicate that CX3CL1 negatively modulates AMPA receptor function at active glutamatergic synapses through cell-signaling pathways by influencing the balance between kinase and phosphatase activity.

Inflammation-Like Glial Response in Lead-Exposed Immature Rat Brain

Numerous studies on lead (Pb) neurotoxicity have indicated this metal to be a dangerous toxin, particularly during developmental stages of higher organisms. Astrocytes are responsible for sequestration of this metal in brain tissue. Activation of astroglia may often lead to loss of the buffering function and contribute to pathological processes. This phenomenon is accompanied by death of neuronal cells and may be connected with inflammatory events arising from the production of a wide range of cytokines and chemokines. The effects of prolonged exposure to Pb upon glial activation are examined in immature rats to investigate this potential proinflammatory effect. When analyzed at the protein level, glial activation is observed after Pb exposure, as reflected by the increased level of glial fibrillary acidic protein and S-100beta proteins in all parts of the brain examined. These changes are associated with elevation of proinflammatory cytokines. Production of interleukin (IL)-1beta and tumor necrosis factor-alpha is observed in hippocampus, and production of IL-6 is seen in forebrain. The expression of fractalkine is observed in both hippocampus and forebrain but inconsiderably in the cerebellum. In parallel with cytokine expression, signs of synaptic damage in hippocampus are seen after Pb exposure, as indicated by decreased levels of the axonal markers synapsin I and synaptophysin. Obtained results indicate chronic glial activation with coexisting inflammatory and neurodegenerative features as a new mechanism of Pb neurotoxicity in immature rat brain.

Nephritogenic anti-DNA antibodies regulate gene expression in MRL/lpr mouse glomerular mesangial cells

OBJECTIVE

Lupus-associated IgG anti-double-stranded DNA antibodies are thought to be pathogenic in the kidney due to cross-reaction with glomerular antigens, leading subsequently to immune complex formation in situ and complement activation. We undertook this study to determine if pathogenic anti-DNA antibodies may also contribute to renal damage by directly influencing mesangial gene expression.

METHODS

Complementary DNA microarray gene profiling was performed in primary mesangial cells (derived from lupus-prone MRL/lpr mice) treated with pathogenic, noncomplexed anti-DNA antibodies. Significant gene up-regulation induced by anti-DNA antibodies as determined by microarray analysis was further investigated by real-time polymerase chain reaction and methods to detect the relevant proteins. Induction of proinflammatory genes by pathogenic antibodies was confirmed by comparing gene expression in glomeruli of old versus young MRL/lpr mice, and by antibody injection in vivo.

RESULTS

Pathogenic, but not nonpathogenic, antibodies significantly induced a number of transcripts, including CXCL1/KC, LCN2, iNOS, CX3CL1/fractalkine, SERPINA3G, and IkappaBalpha ("marker genes"). Blocking of Fcgamma receptors or using Fcgamma chain-knockout mesangial cells had no effect on the gene regulation effect of the pathogenic antibody R4A, indicating a non-Fc-dependent mechanism. The glomerular expression of these marker genes increased over time with the development of glomerular antibody deposition and active nephritis in MRL/lpr mice. Moreover, injection of R4A into SCID mice in vivo significantly up-regulated glomerular marker gene expression.

CONCLUSION

These findings indicate that the renal pathogenicity of anti-DNA antibodies may be attributed in part to their ability to directly modulate gene expression in kidney mesangial cells through both Fc-dependent and non-Fc-dependent mechanisms.

The CC Chemokine MCP-1 Stimulates Surface Expression of CX3CR1 and Enhances the Adhesion of Monocytes to Fractalkine/CX3CL1 via p38 MAPK

The membrane-anchored form of CX3CL1 has been proposed as a novel adhesion protein for leukocytes. This functional property of CX3CL1 is mediated through CX3CR1, a chemokine receptor expressed predominantly on circulating white blood cells. Thus far, it is still uncertain at what stage of the trafficking process CX3CR1 becomes importantly involved and how the CX3CR1-dependent adhesion of leukocytes is regulated during inflammation. The objective of this study was to examine the functional effects of chemokine stimulation on CX3CR1-mediated adhesion of human monocytes. Consistent with previous reports, our data indicate that the activity of CX3CR1 on resting monocytes is sufficient to mediate cell adhesion to CX3CL1. However, the basal, nonstimulated adhesion activity is low, and we hypothesized that like the integrins, CX3CR1 may require a preceding activation step to trigger firm leukocyte adhesion. Compatible with this hypothesis, stimulation of monocytes with MCP-1 significantly increased their adhesion to immobilized CX3CL1, under both static and physiological flow conditions. The increase of the adhesion activity was mediated through CCR2-dependent signaling and obligatory activation of the p38 MAPK pathway. Stimulation with MCP-1 also induced a rapid increase of CX3CR1 protein on the cell surface. Inhibition of the p38 MAPK pathway prevented this increase of CX3CR1 surface expression and blunted the effect of MCP-1 on cell adhesion, indicating a causal link between receptor surface density and adhesion activity. Together, our data suggest that a chemokine signal is required for firm CX3CR1-dependent adhesion and demonstrate that CCR2 is an important regulator of CX3CL1-dependent leukocyte adhesion.

The neuronal chemokine CX3CL1/fractalkine selectively recruits NK cells that modify experimental autoimmune encephalomyelitis within the central nervous system

Leukocyte trafficking to the central nervous system (CNS), regulated in part by chemokines, determines severity of the demyelinating diseases multiple sclerosis (MS) and experimental autoimmune encephalomyelitis (EAE). To examine chemokine receptor CX3CR1 in EAE, we studied CX3CR1(GFP/GFP) mice, in which CX3CR1 targeting by insertion of Green Fluorescent Protein (GFP) allowed tracking of CX3CR1+ cells in CX3CR1(+/GFP) animals and cells destined to express CX3CR1 in CX3CR1(GFP/GFP) knockouts. NK cells were markedly reduced in the inflamed CNS of CX3CR1-deficient mice with EAE, whereas recruitment of T cells, NKT cells and monocyte/macrophages to the CNS during EAE did not require CX3CR1. Impaired recruitment of NK cells in CX3CR1(GFP/GFP) mice was associated with increased EAE-related mortality, nonremitting spastic paraplegia and hemorrhagic inflammatory lesions. The absence of CD1d did not affect the severity of EAE in CX3CR1(GFP/GFP) mice, arguing against a role for NKT cells. Accumulation of NK cells in livers of wild-type (WT) and CX3CR1(GFP/GFP) mice with cytomegalovirus hepatitis was equivalent, indicating that CX3CL1 mediated chemoattraction of NK cells was relatively specific for the CNS. These results are the first to define a chemokine that governs NK cell migration to the CNS, and the findings suggest novel therapeutic manipulation of CX3CR1+ NK cells.

Expression of fractalkine and fractalkine receptor in urinary bladder after cyclophosphamide (CYP)-induced cystitis

Alterations in the expression of the chemokine, fractalkine (CX3CL1), were examined in the urinary bladder after cyclophosphamide (CYP)-induced cystitis of varying duration: acute (4 h or 48 h), or chronic (10 day). CYP-induced cystitis significantly (p<or=0.01) increased fractalkine protein expression in the urinary bladder with acute (48 h) and chronic CYP treatment. Western blot analysis also demonstrated significantly (p<or=0.01) increased fractalkine expression in the whole urinary bladder with acute (1.5-2.2-fold) and chronic (3-fold) CYP-induced cystitis. Immunohistochemistry for fractalkine-immunoreactivity revealed little fractalkine-IR in control or acute (4 h) CYP-treated rat urinary bladders except in a vascular bed but showed no colocalization with nerve fibers in the suburothelial plexus in any experimental group. However, expression was significantly (p<or=0.001) upregulated in the urothelium with 48 h or chronic CYP treatment. Similarly, fractalkine receptor (CX3CR1)-IR was significantly (p<or=0.001) upregulated in the urothelium with 48 h or chronic CYP treatment. These studies demonstrated upregulation of the chemokine, fractalkine, in the urinary bladder and specifically in the urothelium with CYP-induced cystitis. Chemokines, and specifically, fractalkine, may be another class of neuromodulatory agents upregulated in the urinary bladder that can affect micturition function and sensory processing with cystitis and may represent novel, drug targets for cystitis.

T280M variation of the CX3C receptor gene is associated with increased risk for severe respiratory syncytial virus bronchiolitis

BACKGROUND

Recent data suggest that immunologic response during respiratory syncytial virus (RSV) infection is partially modified through interaction of viral G glycoprotein with the host's chemokine receptor, CX3CR1. We hypothesized that two nonsynonymous, single-nucleotide polymorphisms of the CX3CR1 gene (CX3CR1-V249I and CX3CR1-T280M) that disrupt the affinity of CX3CR1 for its natural ligand (fractalkine) could also affect the G glycoprotein-CX3CR1 pathway.

METHODS

To test the hypothesis, DNA samples were obtained from 82 children hospitalized for RSV bronchiolitis in a 1-year period. One hundred twenty sex-matched healthy adults, without a history of severe lower respiratory tract infections, formed the control group.

RESULTS

Epidemiologic data showed an increase in the RSV infection rate during the late winter season, with a peak rate in early spring. Genotyping revealed predominance of the 280M-containing genotypes (M/M or T/M) in cases compared with controls (37.8% versus 20.8%, respectively; odds ratio, 2.03; 95% confidence interval, 1.1-3.9; P = 0.025), demonstrating an association between the common CX3CR1-T280M variations and increased risk of severe RSV bronchiolitis.

CONCLUSIONS

Our findings support the hypothesis of the pivotal role of the G glycoprotein CX3CR1 pathway in the pathogenesis of RSV bronchiolitis and propose CX3CR1 as a potential therapeutic target.

Resveratrol suppresses tumor necrosis factor-alpha-induced fractalkine expression in endothelial cells

Up-regulation of fractalkine is involved in vascular and tissue damage in inflammatory conditions. Resveratrol has been shown to have anti-inflammatory, antioxidant, and antitumor activities. Its regulatory effects on expression of fractalkine in vascular endothelial cells and fractalkine receptor CX3CR1 in monocytes have not been studied. We evaluated the effects of resveratrol on fractalkine expression in human umbilical vein endothelial cells and CX3CR1 expression in THP-1 cells in response to treatment with tumor necrosis factor (TNF)-alpha. TNF-alpha significantly induced fractalkine mRNA and protein expression in endothelial cells. Resveratrol strongly suppressed TNF-alpha-induced fractalkine expression in endothelial cells through suppression of nuclear factor-kappaB and Sp1 activities. Resveratrol decreased the number of TNF-alpha-induced fractalkine-positive endothelial cells and CX3CR1-positive cells determined by flow cytometric analysis. Resveratrol suppressed TNF-alpha-stimulated monocytes adhesion to human umbilical vein endothelial cells. Immunohistochemical analysis revealed that resveratrol suppressed TNF-alpha-induced arterial endothelial fractalkine expression in heart, kidney, and intestine and decreased ED-1-positive cell infiltration in intestinal villi. Resveratrol may provide a new pharmacological approach for suppressing fractalkine/CX3CR1-mediated injury in inflammatory conditions.

Involvement of interaction between Fractalkine and CX3CR1 in cytotoxicity of natural killer cells against tumor cells

Many chemokine receptors are typically found on natural killer cells, including CX3CR1, the receptor for the chemokine fractalkine (FKN). This study explored whether interaction between CX3CR1 and FKN is relevant for NK cell functions in cytotoxicity against tumors. FKN expression was examined by polymerase chain reaction and CX3CR1 expression in NK cells was analyzed by flow cytometry. NK cell cytotoxicity was examined by 4-h 51Cr-release assay. FKN was expressed in a variety of tumor cell lines such as K562 cells, an NK-sensitive cell line. Approximately 90% of peripheral blood NK cells and almost all of the NK cell line, NK-92 cells, expressed CX3CR1. Anti-CX3CR1 antibody strongly neutralized the cytotoxicity of NK cells against K562 cells, and pretreatment of NK cells with recombinant soluble FKN improved the cytolytic function on tumor cells. This study demonstrates that an interaction between CX3CR1 on NK cells and FKN on tumor cells is involved in the natural cytotoxicity of NK cells against tumors.

The chemokines, CX3CL1, CCL14, and CCL4, promote human trophoblast migration at the feto-maternal interface

Human embryo implantation is a complex process involving blastocyst attachment to the endometrial epithelium and subsequent trophoblast invasion of the decidua. Chemokines, critical regulators of leukocyte migration, are abundant in endometrial epithelial and decidual cells at this time. We hypothesized that endometrial chemokines stimulate trophoblast invasion. Chemokine receptors CX3CR1 and CCR1 were immunolocalized in human first-trimester implantation sites, specifically to endovascular extravillous trophoblasts, but not to the invading interstitial EVTs (iEVTs), with weak staining also on syncytium. CCR3 was localized to invading iEVTs and to microvilli on the syncytial surface. Expression of CX3CL1 (fractalkine), CCL7 (MCP-3), and their receptors (CX3CR1, CCR1, CCR2, CCR3, and CCR5) mRNA was examined in cellular components of the maternal-embryonic interface by RT-PCR. Both chemokines were abundant in entire endometrium and placenta, endometrial cells (primary cultures and HES, a human endometrial epithelial cell line) and trophoblast cell lines (JEG-3, ACIM-88, and ACIM-32). Chemokine receptor mRNA was expressed by placenta and trophoblast cell lines: CCR1 by all trophoblast cell types, whereas CCR2, CCR3, and CX3CR1 were more variable. CX3CR1, CCR1, CCR2, and CCR5 were also expressed by endometrial cells. Migration assays used the trophoblast cell line most closely resembling extravillous cytotrophoblast (AC1M-88). Trophoblast migration occurred in response to CX3CL1, CCL14, and CCL4, but not CCL7. Endometrial cell-conditioned media also stimulated trophoblast migration; this was attenuated by neutralizing antibodies to CX3CL1 and CCL4. Thus, chemokines are expressed by maternal and embryonic cells during implantation, whereas corresponding receptors are on trophoblast cells. Promotion of trophoblast migration by chemokines and endometrial cell conditioned medium indicates an important involvement of chemokines in maternal-fetal communication.

The expression of fractalkine in the endometrium during the menstrual cycle

OBJECTIVE

The objective of the study was to evaluate the presence of fractalkine in the endometrium of the uterus and the change of fractalkine protein levels during menstrual cycle.

METHODS

Twelve samples of endometrium of the uterus were obtained from gynecological patients who underwent total hysterectomy. Western blotting, RT-PCR and immunohistochemistry were performed.

RESULTS

Fractalkine protein was detected in the endometrium of the uterus. Positive staining was confirmed in the epithelial cells and grandular cells in the endometrium. Expression levels of fractalkine protein and mRNA in the endometrium during secretory phase were significantly higher than those during proliferative phase. Immunohistochemical analysis using an anti-CX3CR1 antibody demonstrated positive staining in the glandular cells of the endometrium of the uterus.

CONCLUSION

Fractalkine was expressed in the endometrium and its production was up-regulated during secretory phase.

Identification of the lacZ insertion site and beta-galactosidase expression in transgenic chickens

The quail:chick chimera system is a classical research model in developmental biology. An improvement over the quail:chick chimera system would be a line of transgenic chickens expressing a reporter gene. Transgenic chickens carrying lacZ and expressing bacterial beta-galactosidase have been generated, but complete characterization of the insertion event and characterization of beta-galactosidase expression have not previously been available. The genomic sequences flanking the retroviral insertion site have now been identified by using inverse polymerase chain reaction (PCR), homozygous individuals have been identified by using PCR-based genotyping, and beta-galactosidase expression has been evaluated by using Western analysis and histochemistry. Based upon the current draft of the chicken genome, the viral insertion carrying the lacZ gene has been located on chromosome 11 within the predicted gene for neurotactin/fractalkine (CX3CL1); neurotactin mRNA expression appears to be missing from the brain of homozygous individuals. When Generation 2 (G2) lacZ-positive individuals were inter-mated, they generated 361 G3 progeny; 82 were homozyous for lacZ (22.7%), 97 were wild-type non-transgenic (26.9%), and 182 (50.4%) were hemizygous for lacZ. Western analysis revealed the highest expression in the muscle and liver. With the identification of homozygous birds, the line of chickens is now designated NCSU-Blue1.

Inhibition of CX3CL1 (fractalkine) improves experimental autoimmune myositis in SJL/J mice

Idiopathic inflammatory myopathy is a chronic inflammatory muscle disease characterized by mononuclear cell infiltration in the skeletal muscle. The infiltrated inflammatory cells express various cytokines and cytotoxic molecules. Chemokines are thought to contribute to the inflammatory cell migration into the muscle. We induced experimental autoimmune myositis (EAM) in SJL/J mice by immunization with rabbit myosin and CFA. In the affected muscles of EAM mice, CX3CL1 (fractalkine) was expressed on the infiltrated mononuclear cells and endothelial cells, and its corresponding receptor, CX3CR1, was expressed on the infiltrated CD4 and CD8 T cells and macrophages. Treatment of EAM mice with anti-CX3CL1 mAb significantly reduced the histopathological myositis score, the number of necrotic muscle fibers, and infiltration of CD4 and CD8 T cells and macrophages. Furthermore, treatment with anti-CX3CL1 mAb down-regulated the mRNA expression of TNF-alpha, IFN-gamma, and perforin in the muscles. Our results suggest that CX3CL1-CX3CR1 interaction plays an important role in inflammatory cell migration into the muscle tissue of EAM mice. The results also point to the potential therapeutic usefulness of CX3CL1 inhibition and/or blockade of CX3CL1-CX3CR1 interaction in idiopathic inflammatory myopathy.

Increased expression of the chemokine fractalkine in Crohn's disease and association of the fractalkine receptor T280M polymorphism with a fibrostenosing disease Phenotype

BACKGROUND

The fractalkine receptor CX3CR1 has been shown to be involved in inflammation and immune response. Recently, two polymorphisms of CX3CR1 (V249I and T280M) were reported.

AIMS

Our aim was to analyze fractalkine expression and the role of CX3CR1 polymorphisms in Crohn's disease (CD).

METHODS

We determined fractalkine mRNA expression in the intestinal epithelial cell (IEC) line SW480 after stimulation with proinflammatory cytokines as well as in inflamed (n = 14) and noninflamed (n = 14) CD lesions by quantitative PCR. By restriction fragment length polymorphism analysis, genomic DNA from 206 patients with CD and 211 unrelated controls was analyzed for the two single nucleotide polymorphisms in the CX3CR1 gene, which result in the V249I and T280M substitutions.

RESULTS

All proinflammatory stimuli (TNF-alpha, IL-1beta, LPS) significantly increased fractalkine mRNA expression in IEC. There was also a significant increase in fractalkine mRNA expression in inflamed lesions of CD patients when compared to noninflamed colonic mucosa (p = 0.02). Intestinal fractalkine mRNA levels correlated highly with IL-8 mRNA expression levels (r = 0.931). However, there was no difference in the V249I and T280M genotype frequencies between CD patients and the control group. In the CD group, 33.0% were heterozygous and 8.3% homozygous for the V249I polymorphism, while 23.3% were heterozygous and 4.4% homozygous for the T280M polymorphism. All T280M homozygotes were diagnosed of intestinal stenosis (p = 0.03 vs wildtype and heterozygous carriers) and had significantly more often ileocolonic involvement more often than patients with wildtype and heterozygous genotypes (p = 0.01). These associations were independent of the CARD15 genotype status.

CONCLUSIONS

The expression of the chemokine fractalkine is upregulated by proinflammatory cytokines and enhanced in inflamed CD lesions. The CX3CR1 T280M polymorphism appears to influence CD phenotype and localization.

The Amino Terminus and the Third Extracellular Loop of CX3CR1 Contain Determinants Critical for Distinct Receptor Functions

The G protein-coupled receptor CX3CR1 is a specific receptor for the CX3C chemokine fractalkine (CX3CL1 according to the new chemokine nomenclature). The aim of this study was to identify receptor elements that contribute independently to agonist binding and receptor activation. Targeted mutation of selected acidic amino acid residues demonstrated that the binding activity of CX3CR1 was critically dependent on the two negatively charged residues Asp25 and Glu254 located on the N-terminal domain and third extracellular loop, respectively. In addition, mutation of the uncharged polar residue Tyr14 in the amino terminus caused a reduction in the ligand binding affinity. In contrast, the three acidic residues Glu13, Asp16, and Asp266 did not contribute to ligand binding but were crucial for receptor activation. The mutant receptors E13A, D16A, and D266A bound fractalkine with high affinity but were unable to induce signaling events necessary to support chemotaxis. These acidic residues may engage in electrostatic interactions with basic residues on fractalkine that are necessary for receptor function but not for binding. Our data are consistent with a model of chemokine receptor activation consisting of a multi-step mechanism. Step one mediates the high-affinity fractalkine binding involving Tyr14, Asp25, and Glu254. The initial interaction then triggers the engagement of Glu13, Asp16, and Asp266, which are necessary for CX3CR1 activation.

Autocrine role of vascular IL-15 in intimal thickening

Interleukin 15 (IL-15) is a pro-inflammatory cytokine that modulates T cell recruitment and activation, independent of antigen. It has been detected in human atherosclerotic plaques and atherosclerotic plaques of apoE-/- mice. IL-15 regulates fractalkine (FKN)-CX3CR1 chemokine signaling which is involved in atherogenesis and promotes SMC proliferation. We investigated the role of IL-15 in intimal thickening after arterial injury. Treatment of serum-stimulated SMC with IL-15 in vitro attenuated proliferation and suppressed CX3CR1 and FKN mRNA expression. The role of endogenous IL-15 in vivo was investigated in injured carotid arteries of mice. Periadventitial arterial injury resulted in increased IL-15 expression in the media and neointima, paralleled by increased IL-15 receptor alpha expression. Blockade of endogenous IL-15 increased intimal thickening. FKN and CX3CR1 expression increased after injury and were further augmented after IL-15 blockade. These data suggest that endogenous IL-15 attenuated intimal thickening after arterial injury. The potential mechanism of action is suppression of CX3CR1 signaling.

Advanced glycation end-product induces fractalkine gene upregulation in normal rat glomeruli

BACKGROUND

We previously reported that fractalkine was upregulated in streptozotocin-induced diabetic kidneys. Fractalkine in diabetic kidneys was detected on glomerular capillaries and the mesangium. This upregulation was suppressed by treatment with angiotensin-converting enzyme inhibitor (ACE-I) or aminoiguanidine. We examined what factors induce fractalkine upregulation in normal rat glomeruli.

METHODS

Glomeruli were collected from the kidneys of normal Sprague-Dawley rats by a microdissection method. Ten glomeruli were incubated in a solution with glucose, mannitol, angiotensin II, tumour necrosis factor (TNF)-alpha and advanced glycation end-product (AGE)-bovine serum albumin (BSA) for 1, 2 and 4 h. Fractalkine mRNA expression in glomeruli was examined by reverse transcription-polymerase chain reaction.

RESULTS

Fractalkine mRNA levels in the 30 mM glucose solution significantly increased (121%) compared with those in the control or 30 mM mannitol solution at 1 h. Fractalkine mRNA levels in the 15 mM glucose solution showed no significant differences at 1 or 2 h, but significantly increased (106%) after 4 h incubation. Fractalkine mRNA levels in 10(-6)-10(-8) M angiotensin II solution showed no significant differences. Fractalkine mRNA levels in the 5 or 10 ng/ml TNF-alpha solution significantly increased compared with those in the control in a time- and dose-dependent manner (by 94 to 253%). Fractalkine mRNA levels in the 50-200 microg/ml AGE-BSA solution also increased compared with those in BSA solution in a time- and dose-dependent manner (by 119 to 261%). By pre-incubation with MG132, a nuclear factor-kappaB inhibitor, fractalkine upregulation by AGE-BSA or 30 mM glucose was completely suppressed.

CONCLUSIONS

High glucose levels, AGE formation and cytokine activation in diabetes may induce fractalkine upregulation in the kidneys and lead to progression of diabetic nephropathy.

Chemokine CX3CL1 protects rat hippocampal neurons against glutamate-mediated excitotoxicity

Excitotoxicity is a cell death caused by excessive exposure to glutamate (Glu), contributing to neuronal degeneration in many acute and chronic CNS diseases. We explored the role of fractalkine/CX3CL1 on survival of hippocampal neurons exposed to excitotoxic doses of Glu. We found that: CX3CL1 reduces excitotoxicity when co-applied with Glu, through the activation of the ERK1/2 and PI3K/Akt pathways, or administered up to 8 h after Glu insult; CX3CL1 reduces the Glu-activated whole-cell current through mechanisms dependent on intracellular Ca2+; CX3CL1 is released from hippocampal cells after excitotoxic insult, likely providing an endogenous protective mechanism against excitotoxic cell death.

Induction of CX3CL1 expression in astrocytes and CX3CR1 in microglia in the spinal cord of a rat model of neuropathic pain

Previous studies have shown that chemokines might play a role in the pathology of chronic pain. The purpose of this study was to provide an immunohistochemical description of the distribution of CX3CL1 (fractalkine) and its receptor CX3CR1 in the rat spinal cord in a model of inflammatory pain induced by unilateral intraplantar complete Freund's adjuvant (CFA) and in a model of neuropathic pain induced by L5 spinal nerve ligation (modified Chung model or mSNL). In naïve rats, CX3CL1 is found in the cytoplasm of neurons as shown by colocalization of CX3XL1 and NeuN. Similar distribution of CX3CL1 was observed after CFA, whereas after mSNL, CX3CL1 was not only observed in neurons but also found in astrocytes, as shown by colocalization of CX3CL1 and GFAP. Weak immunoreactivity for the CX3CL1 receptor, CX3CR1, was found in microglia in the spinal cord of either naïve rats or rats with inflammation. However, after spinal nerve injury, CX3CR1-LI was upregulated in microglia throughout the dorsal horn.

PERSPECTIVE

This study shows that spinal nerve injury, but not peripheral inflammation, induces the expression of a chemokine, CX3CL1 (fractalkine), in astrocytes and upregulates CX3CR1 in microglia in the spinal cord. This selective regulation of CX3CL1 and its receptor, CX3CR1, suggests that these chemokines may represent new targets for the treatment of neuropathic pain.

IFN-gamma, produced by NK cells that infiltrate liver allografts early after transplantation, links the innate and adaptive immune responses

The role of NK cells following solid organ transplantation remains unclear. We examined NK cells in acute allograft rejection using a high responder model (DA-->Lewis) of rat orthotopic liver transplantation. Recipient-derived NK cells infiltrated liver allografts early after transplantation. Since chemokines are important in the trafficking of cells to areas of inflammation, we determined the intragraft expression of chemokines known to attract NK cells. CCL3 was significantly increased in allografts at 6 h post-transplant as compared to syngeneic grafts whereas CCL2 and CXCL10 were elevated in both syngeneic and allogeneic grafts. CXCL10 and CX3CL1 were significantly upregulated in allografts by day 3 post-transplant as compared to syngeneic grafts suggesting a role for these chemokines in the recruitment of effector cells to allografts. Graft-infiltrating NK cells were shown to be a major source of IFN-gamma, and IFN-gamma levels in the serum were markedly increased, specifically in allograft recipients, by day 3 post-transplant. Accordingly, in the absence of NK cells the levels of IFN-gamma were significantly decreased. Furthermore, graft survival was significantly prolonged. These data suggest that IFN-gamma-producing NK cells are an important link between the innate and adaptive immune responses early after transplantation.

Expression of CX3C chemokine, fractalkine, and its receptor CX3CR1 in experimental autoimmune anterior uveitis

PURPOSE

To demonstrate the expression and location of CX3C chemokine, fractalkine, and its receptor, CX3CR1, in the iris/ciliary body and thus establish their roles in experimental autoimmune anterior uveitis, an animal model of human acute anterior uveitis.

METHODS

Uveitis was induced in Lewis rats by injection of melanin associated antigen into the peritoneum and footpad. At defined times, fractalkine and its receptor CX3CR1 mRNA expression in the iris/ciliary body were measured by using a semiquantitative polymerase chain reaction method. Fractalkine in aqueous humor was determined by enzyme linked immunosorbent assay. The cellular sources of fractalkine were determined by immunhistochemical staining. In a separate experiment, NF-kappaB inhibitor, pyrrolidine dithiocarbamate (PDTC; 200 mg/kg/day) was administrated intraperitoneally daily after immunization. The rats were sacrificed on day 14 of immunization. Fractalkine mRNA in iris/ciliary body and fractalkine concentration in aqueous humor were determined after PDTC treatment.

RESULTS

Fractalkine mRNA was found to be upregulated in the iris/ciliary body nine days after immunization, preceding clinical disease onset. CX3CR1 mRNA exhibited peak levels at day 14, coincident with disease onset. Fractalkine in aqueous humor showed an expression profile similar to mRNA expression. PDTC (200 mg/kg) markedly inhibited the expression of fractalkine mRNA in the iris/ciliary body, and fractalkine protein in aqueous humor. Immunohistochemical staining revealed that fractalkine was expressed on vascular endothelial cells and infiltrated inflammatory cells. Treatment with PDTC significantly reduced both the number of leukocyte infiltrations in the iris/ciliary body and fractalkine expression on vascular endothelial cells.

CONCLUSIONS

The sequential expression of fractalkine may direct distinct CX3CR1 receptor expressing mononuclear cell subsets to inflammatory sites. Fractalkine expression is modulated, at least in part, through the NF-kappaB signaling pathway. These findings provide new insight into the molecular mechanisms of acute anterior uveitis and suggest fractalkine or NF-kappaB as a new drug target for uveitis therapy.

Metalloproteinase inhibitors for the disintegrin-like metalloproteinases ADAM10 and ADAM17 that differentially block constitutive and phorbol ester-inducible shedding of cell surface molecules

The transmembrane metzinkin-proteases of the ADAM (a disintegrin and a metalloproteinase)-family ADAM10 and ADAM 17 are both implicated in the ectodomain shedding of various cell surface molecules including the IL6-receptor and the transmembrane chemokines CX3CL1 and CXCL16. These molecules are constitutively released from cultured cells, a process that can be rapidly enhanced by cell stimulation with phorbol esters such as PMA. Recent research supports the view that the constitutive cleavage predominantly involves ADAM10 while the inducible one is mediated to a large extent by ADAM17. We here describe the discovery of hydroxamate compounds with different potency against ADAM10 and ADAM17 and different ability to block constitutive and inducible cleavage of IL6R, CX3CL1 and CXCL16 by the two proteases. By screening a number of hydroxamate inhibitors for the inhibition of recombinant metalloproteinases, a compound was found inhibiting ADAM10 with more than 100-fold higher potency than ADAM17, which may be explained by an improved fit of the compound to the S1' specificity pocket of ADAM10 as compared to that of ADAM17. In cell-based cleavage experiments this compound (GI254023X) potently blocked the constitutive release of IL6R, CX3CL1 and CXCL16, which was in line with the reported involvement of ADAM10 but not ADAM17 in this process. By contrast, the compound did not affect the PMA-induced shedding, which was only blocked by GW280264X, a potent inhibitor of ADAM17. As expected, GI254023X did not further decrease the residual release of CX3CL1 and CXCL16 in ADAM10-deficient cells verifying that the compound's effect on the constitutive shedding of these molecules was exclusively due to the inhibition of ADAM10. Thus, GI254023X may by of use as a preferential inhibitor of constitutive shedding events without effecting the inducible shedding in response to agonists acting similar to PMA.

Recycling of the Membrane-anchored Chemokine, CX3CL1

CX(3)CL1 (fractalkine) plays an important role in inflammation by acting as both chemoattractant and as an adhesion molecule. As for other chemokines, expression of CX(3)CL1 is known to be regulated at the level of transcription and translation. The unique transmembrane structure of CX(3)CL1 raises the possibility of additional functional regulation by altering its abundance at the cell surface. This could be accomplished in principle by changes in traffic between subcellular compartments. To analyze this possibility we examined the subcellular distribution of CX(3)CL1 in human ECV-304 cells stably expressing untagged or green fluorescent protein-tagged forms of the chemokine. CX(3)CL1 was present in two distinct compartments, diffusely on the plasma membrane and in a punctate juxtanuclear compartment. The latter shared some features with, yet was distinct from the conventional endocytic pathway and may represent a specialized recycling subcompartment. Accordingly, surface CX(3)CL1 was found to be in dynamic equilibrium with the juxtanuclear vesicular compartment. Intracellular CX(3)CL1 co-localized with the SNARE (soluble N-ethylmaleimide factor attachment protein receptor) proteins syntaxin-13 and VAMP-3. Cleavage of VAMP-3 by tetanus toxin or impairment of syntaxin-13 function by expression of a dominant-negative allele inhibited the ability of internalized CX(3)CL1 to traffic back to the plasma membrane. These data demonstrate the existence of a dynamic, SNARE-mediated recycling of CX(3)CL1 from the cell surface to and from an endomembrane storage compartment. The intracellular storage depot may serve as a source of the chemokine that could be rapidly mobilized by stimuli.

Fractalkine Targeting with a Receptor-Mimicking Peptide-Amphiphile

In this study we have designed the NTFR peptide-amphiphile that mimics a fragment of the N-terminus of the fractalkine receptor (CX(3)CR1) and specifically targets fractalkine, a novel adhesion molecule expressed on the surface of inflamed endothelial cells. Bioartificial membranes were constructed from mixtures of NTFR peptide-amphiphiles and DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) phospholipids, and the affinity and specificity of fractalkine for the synthetic NTFR was investigated with an atomic force microscope (AFM). Fractalkine was immobilized onto the AFM tips, and forces were collected between fractalkine and the bioartificial membranes. The adhesive interactions were studied at the collective level, when each adhesion event corresponded to the rupture of multiple biomolecular bonds. Retraction force profiles for the fractalkine-NTFR system exhibited single or multiple peaks and a small percentage of the force curves demonstrated stretching of the fractalkine-NTFR complex. Strong adhesion was measured when both DPPC and NTFR were present, compared to pure NTFR surfaces. This may be due to the fact that the DPPC molecule is shorter, and thus it can provide more space for the peptide headgroup to bend and expose its sequence at the interface. Specificity was demonstrated by comparing the NTFR-fractalkine adhesion to the forces between the alpha(5)beta(1) integrin (an adhesion receptor expressed on the surface of endothelial cells) and other surfaces such as GRGDSP (the specific ligand for alpha(5)beta(1)), GRGESP (an inactive sequence), and NTFR.