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

The importance of the intracellular Ca concentration ([Ca]) in neutrophil function has been intensely studied. However, the role of the intracellular Na concentration ([Na]) which is closely linked to the intracellular Ca regulation has been largely overlooked. The [Na] is regulated by Na transport proteins such as the Na/Ca-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]. fMLF induces a sustained increase of [Na], 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]. The difference in [Na] measured at = 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] 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] by a few mmol/l may suffice to switch its transport mode from forward (Ca-efflux) to reverse (Ca-influx) mode. The role of NCX1 in neutrophil chemotaxis is corroborated by its blocker, which also causes a complete inhibition of chemotaxis.