Osteopontin modulates CD44‐dependent chemotaxis of peritoneal macrophages through G‐protein‐coupled receptors: Evidence of a role for an intracellular form of osteopontin

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Abstract
Expression of osteopontin (OPN) by activated T-cells and macrophages is required for the development of cell-mediated inflammatory responses. Acting through integrin αvβ3 and CD44 receptors, OPN can promote chemoattraction and pro-inflammatory cytokine expression by macrophages. In this study, we have used periotoneal macrophages from OPN−/, CD44−/−, and WT mice to study the relationship between OPN and CD44 in macrophage migration. Using confocal microscopy, we show that OPN co-distributes with CD44 inside macrophages at cell edges and in cell processes in a mutually dependent manner. The existence of an intracellular form of OPN is supported by pulse-chase studies in which a thrombin-sensitive, phosphorylated protein immunoprecipitated with OPN antibodies is retained inside macrophages. In OPN−/− and CD44−/− macrophages, the absence of CD44 and OPN, respectively, is associated with the formation of fewer cell processes, reduced cell fusion required to form functional multinucleated osteoclasts in the presence of CSF-1 and RANKL, and impaired chemotaxis. Whereas the chemotaxis of CD44−/− cells to various chemoattractants is almost completely abrogated, a differential effect is seen with the OPN−/− cells. Thus, OPN−/− cells migrate normally towards CSF-1 but not towards fMLP and MCP-1, which signal through G-protein coupled receptors (GPCRs). That the GPCR-mediated migration is dependent upon the level of cell-surface CD44 is indicated by the reduced cell-surface expression of CD44 in OPN−/− cells and a comparable impairment in the chemotaxis of CD44+/− cells. Although chemotaxis of OPN−/− cells could be rescued by an OPN substratum, or by addition of high levels of OPN in solution, no response is evident with physiological levels of OPN, indicating a requirement for the CD44-associated intracellular OPN in CD44 cell-surface expression. These studies indicate, therefore, that the level of cell surface CD44 is critical for GPCR-mediated chemotaxis by peritoneal macrophages and suggest that a novel intracellular form of OPN may modulate CD44 activities involved in these processes. J. Cell. Physiol. 198: 155–167, 2004.