Role of the Intracellular Sodium Homeostasis in Chemotaxis of Activated Murine Neutrophils

The importance of the intracellular Ca(2+) concentration ([Ca(2+)](i)) in neutrophil function has been intensely studied. However, the role of the intracellular Na(+) concentration ([Na(+)](i)) which is closely linked to the intracellular Ca(2+) regulation has been largely overlooked. The [Na(+)](i) is regulated by Na(+) transport proteins such as the Na(+)/Ca(2+)-exchanger (NCX1), Na(+)/K(+)-ATPase, and Na(+)-permeable, transient receptor potential melastatin 2 (TRPM2) channel. Stimulating with either N-formylmethionine-leucyl-phenylalanine (fMLF) or complement protein C5a causes distinct changes of the [Na(+)](i). fMLF induces a sustained increase of [Na(+)](i), surprisingly, reaching higher values in TRPM2(−/−) neutrophils. This outcome is unexpected and remains unexplained. In both genotypes, C5a elicits only a transient rise of the [Na(+)](i). The difference in [Na(+)](i) measured at t = 10 min after stimulation is inversely related to neutrophil chemotaxis. Neutrophil chemotaxis is more efficient in C5a than in an fMLF gradient. Moreover, lowering the extracellular Na(+) concentration from 140 to 72 mM improves chemotaxis of WT but not of TRPM2(−/−) neutrophils. Increasing the [Na(+)](i) by inhibiting the Na(+)/K(+)-ATPase results in disrupted chemotaxis. This is most likely due to the impact of the altered Na(+) homeostasis and presumably NCX1 function whose expression was shown by means of qPCR and which critically relies on proper extra- to intracellular Na(+) concentration gradients. Increasing the [Na(+)](i) by a few mmol/l may suffice to switch its transport mode from forward (Ca(2+)-efflux) to reverse (Ca(2+)-influx) mode. The role of NCX1 in neutrophil chemotaxis is corroborated by its blocker, which also causes a complete inhibition of chemotaxis.