The chemokine receptor CCR2 mediates the binding and internalization of monocyte chemoattractant protein-1 along brain microvessels

Previous results from this laboratory revealed the presence of high-affinit y saturable binding sites for monocyte chemoattractant protein-1 (MCP-1) al ong human brain microvessels (Andjelkovic et al., 1999; Andjelkovic and Pac hter, 2000), which suggested that CC chemokine receptor 2 (CCR2), the recog nized receptor for this chemokine, was expressed by the brain microvascular endothelium. To test the role of CCR2 directly in mediating MCP-1 interact ions with the brain microvasculature, we assessed MCP-1 binding activity in murine brain microvessels isolated from wild-type mice and from CCR2 (-/-) mice engineered to lack this receptor. Results demonstrate that MCP-1 bind ing is greatly attenuated in microvessels prepared from CCR2 (-/-) mice com pared with wild-type controls. Moreover, microvessels from wild-type mice e xhibited MCP-1-induced downmodulation in MCP-1 binding and a recovery of bi nding activity that was not dependent on de novo protein synthesis. Further more, MCP-1 was shown to be internalized within wild-type microvessels, but not within microvessels obtained from CCR2 (-/-) mice, additionally demons trating that CCR2 is obligatory for MCP-1 endocytosis. Last, internalizatio n of MCP-1, but not transferrin, was observed to be inhibited by disruption of caveolae. Internalized MCP-1 also colocalized at some sites with caveol in-1, a major protein of caveolae, implying that this chemokine is endocyto sed, in part, via nonclathrin-coated vesicles. These results prompt conside ration that MCP-1 signals may be relayed across the blood-brain barrier by highly specialized interactions of this chemokine with its cognate receptor , CCR2, along brain microvascular endothelial cells.