title: Expression and targeting of CX3CL1 (fractalkine) in renal tubular epithelial cells.
abstract: 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.
authors.affiliation: Division of Nephrology, Hospital for Sick Children, 555 University Avenue, Toronto, ON, Canada.
authors.forename: Anne M
authors.initials: AM
authors.lastname: Durkan
authors.forename: R Todd
authors.initials: RT
authors.lastname: Alexander
authors.forename: Guang-Ying
authors.initials: GY
authors.lastname: Liu
authors.forename: Min
authors.initials: M
authors.lastname: Rui
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Femia
authors.forename: Lisa A
authors.initials: LA
authors.lastname: Robinson
mesh_terms: D000818:Animals; D001483:Base Sequence; D002448:Cell Adhesion; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D016764:Cell Polarity; D054428:Chemokine CX3CL1; D020523:Chemokines, CX3C / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D017931:DNA Primers / Q000235:genetics; D004285:Dogs; D004847:Epithelial Cells / Q000166:cytology / Q000378:metabolism; D036681:Fluorescence Recovery After Photobleaching; D015870:Gene Expression; D006031:Glycosylation; D049452:Green Fluorescent Proteins / Q000235:genetics / Q000378:metabolism; D006801:Humans; D007684:Kidney Tubules / Q000166:cytology / Q000378:metabolism*; D007962:Leukocytes / Q000166:cytology; D008565:Membrane Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism
chemical_list: C515538:CX3CL1 protein, human; D054428:Chemokine CX3CL1; D020523:Chemokines, CX3C; D017931:DNA Primers; D008565:Membrane Proteins; D011993:Recombinant Fusion Proteins; D049452:Green Fluorescent Proteins