Salt-dependent chemotaxis of macrophages

Abstract

Macrophages exert a prominent function in immune system host defense, but in the recent years increasing evidence emerged that these cells are in addition potent regulators of salt balance. At the outset, this work was based on previous findings that had demonstrated an accumulation of macrophages in the skin tissue of rats, which had been fed on a high salt diet. The question arose whether this was possibly due to a chemotactic response of the macrophages to the hypertonic environment of skin that had sequestered high amounts of Na⁺ to the interstitial glycosaminoglycans<br /> Chemotaxis is an essential process of immune defense to attract immune cells to sites of pathogenic infection. Monocytes/macrophages have been described to recognize many substances among which are chemokines, bacterial components, complement factors and leukotrienes as chemoattractive signals. In contrast, cell migration toward a hypertonic NaCl stimulus, which might represent a potential harmful environment for the cell by causing hypertonic stress, is a completely novel concept. <br /> In vitro transwell migration assays revealed that RAW264.7 macrophages, peritoneal macrophages and bone marrow-derived macrophages, but not bone marrow-derived dendritic cells show salt-dependent chemotaxis toward of a hypertonic NaCl stimulus. This dose-dependent migration response was specific to hypertonicity by excess NaCl, as it could not be induced by other osmo-active agents like urea or mannitol. Subsequently, the underlying molecular mechanism of salt-dependent chemotaxis was investigated with respect to early and late events in cell migration. Many potential candidates were addressed, demonstrating that a hypertonic NaCl stimulus did not directly affect actin cytoskeleton reorganization and was unable to induce expression of chemoattractive CCL2 or LTB4 in RAW264.7 cells. Furthermore, migration toward excess NaCl was abrogated by the presence of cycloheximide and pertussis toxin, indicating a dependence on protein synthesis and Gαi-coupled GPCRs, respectively. While the osmoprotective transcription factor TonEBP is a central regulator in macrophages for the removal of excess Na⁺ in the interstitium, it was not required for salt-dependent chemotaxis.<br /> Although the complete underlying signaling pathway could not be elucidated, the participation of a NaCl-induced chemotactic factor that acts in an autocrine/ paracrine way, similar to swarming migration behavior of neutrophils, might underlie theses observations. In addition, the role of sodium ion channels in salt-dependent chemotaxis should be considered. Taken together, it is proposed that salt-dependent chemotaxis plays a crucial role in clearance of excess salt in vivo and that any defects in macrophage migration toward areas of high salt storage might result in development of hypertension, as recent studies in rodents and also humans imply.