Expression and Targeting of CX3CL1 (Fractalkine) in Renal Tubular Epithelial Cells

Abstract

The chemokine CX 3 CL1 plays a key role in glomerulonephritis and can act as both chemoattractant and adhesion molecule. CX 3 CL1 also is upregulated in tubulointerstitial injury, but little is known about the subcellular distribution and function of CX 3 CL1 in renal tubular epithelial cells (RTEC). Unexpectedly, it was found that CX 3 CL1 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 CX 3 CL1 in polarized RTEC, MDCK cells that expressed untagged or green fluorescent protein-tagged CX 3 CL1 were generated. The chemokine was present on the apical membrane and in subapical vesicles. Apical targeting of CX 3 CL1 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 CX 3 CL1 is immobile in the apical membrane. However, CX 3 CL1 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 CX 3 CL1 mobile. For exploration of potential functions of apical CX 3 CL1, binding of CX 3 CR1-expressing leukocytes to polarized RTEC was examined. Leukocyte adhesion to the luminal surface was enhanced significantly when CX 3 CL1 was present. These data demonstrate that CX 3 CL1 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.

Fractalkine–CX3CR1-dependent recruitment and retention of human CD1c+ myeloid dendritic cells by in vitro–activated proximal tubular epithelial cells

“…Previous human studies have demonstrated in vivo expression of fractalkine protein by renal tubular epithelial cells. [16][17][18]20 However, we extend these reports by providing in vivo evidence that PTECs, identified on the basis of their expression of aquaporin-1, are a major source of fractalkine in fibrotic kidney disease. Fractalkine expression by human PTECs in vitro has been shown to be induced by protein overload 43 and TNF-a.…”

“…19 Expression of fractalkine at tubulointerstitial sites is localized to endothelial cells of the peritubular capillary network and tubular epithelial cells, with cellular infiltrates detected adjacent to fractalkine-expressing tubular epithelial cells. [16][17][18]20 However, the proximal or distal origin of these fractalkine-expressing cells has not been defined.…”

Chemokines in Renal Injury

“…[77][78][79] Fractalkine/CX3CL1 and its receptor CX3CR are upregulated during renal injury in several mouse models, including experimental folic acid nephropathy, crescentic GN, and diabetic nephropathy. 80,81,78,82,83,77,9 Treatment with a neutralizing antibody against fractalkine/CX3CL1 improves renal damage by blocking crescentic formation and macrophage infiltration in rat nephrotoxic nephritis. 84 All these studies imply there might be an important role for CCL and CX3CL chemokines in chronic renal disease regardless of inciting events.…”

CX3CR1 Reduces Kidney Fibrosis by Inhibiting Local Proliferation of Profibrotic Macrophages

“…There is evidence that the survival of monocytes and tissue macrophages depends on CX 3 CR1 (31)(32)(33), and this required interaction with cell surface-bound CX 3 CL1 (34). CX 3 CL1 + cells have been shown to be present within glomeruli and in the tubular compartment (35), but their exact identity is unclear.…”