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

Karolina Najder,Sandra Schimmelpfennig,Micol Rugi,Albrecht Schwab,Sarah Sargin,Julia Schröder,Leonie Oster,Mégane Lebel,Zoltán Pethő,Etmar Bulk

doi:10.3389/fimmu.2020.02124

Abstract

The importance of the intracellular Ca2+ concentration ([Ca2+]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 Ca2+ regulation has been largely overlooked. The [Na+]i is regulated by Na+ transport proteins such as the Na+/Ca2+-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 (Ca2+-efflux) to reverse (Ca2+-influx) mode. The role of NCX1 in neutrophil chemotaxis is corroborated by its blocker, which also causes a complete inhibition of chemotaxis.

Highlights

Neutrophil migration, adhesion, neutrophil extracellular trap formation, bacterial killing, and production of reactive oxygen species (ROS) require coordinated ion fluxes [1,2,3,4,5,6]
We show that neutrophil chemotaxis is modulated by the extra- and intracellular Na+ concentration
It is evident that the earlier concentration rise leads to higher [Na+]i after 10 min in transient receptor potential melastatin 2 (TRPM2)−/− neutrophils

Summary

Introduction

Neutrophil migration, adhesion, neutrophil extracellular trap formation, bacterial killing, and production of reactive oxygen species (ROS) require coordinated ion fluxes [1,2,3,4,5,6]. Ca2+ is among the best studied ions in this context [7, 8] It is a potent, multifunctional second messenger and its increase in the cytoplasm is a hallmark of neutrophil activation. An increase of the intracellular Ca2+ concentration ([Ca2+]i) is caused by release of Ca2+ from intracellular stores and by receptor- or store-operated Ca2+ entry (ROCE or SOCE, respectively). A gradient of the [Ca2+]i was shown to be modulated by TRPC1 channel in murine neutrophils [4]. Neutrophil activation with fMLF leads to an increase of the intracellular Na+ concentration ([Na+]i) [16]. In inflammation and tumor the [Na+]o is usually increased [19, 20]

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Results

Conclusion