Abstract
Macrophages are highly plastic and dynamic cells that exert much of their function through phagocytosis. Phagocytosis depends on a coordinated, finely tuned, and compartmentalized regulation of calcium concentrations. We examined the role of mitochondrial calcium uptake and mitochondrial calcium uniporter (MCU) in macrophage polarization and function. In primary cultures of human monocyte-derived macrophages, calcium uptake in mitochondria was instrumental for alternative (M2) macrophage polarization. Mitochondrial calcium uniporter inhibition with KB-R7943 or MCU knockdown, which prevented mitochondrial calcium uptake, reduced M2 polarization, while not affecting classical (M1) polarization. Challenging macrophages with E. coli fragments induced spikes of mitochondrial calcium concentrations, which were prevented by MCU inhibition or silencing. In addition, mitochondria remodelled in M2 macrophages during phagocytosis, especially close to sites of E. coli internalization. Remarkably, inhibition or knockdown of MCU significantly reduced the phagocytic capacity of M2 macrophages. KB-R7943, which also inhibits the membrane sodium/calcium exchanger and Complex I, reduced mitochondria energization and cellular ATP levels, but such effects were not observed with MCU silencing. Therefore, phagocytosis inhibition by MCU knockdown depended on the impaired mitochondrial calcium buffering rather than changes in mitochondrial and cellular energy status. These data uncover a new role for MCU in alternative macrophage polarization and phagocytic activity.
Highlights
One of the most important biological functions of macrophages is phagocytosis
We examined the role of mitochondrial calcium uptake and mitochondrial calcium uniporter (MCU) in macrophage polarization and function
To confirm that KB-R7943 effectively inhibited mitochondrial calcium uptake, we imaged resting macrophages pre-loaded with Rhod-2 and Fluo-4 with or without KB-R7943 pre-treatment, during stimulation with ionomycin
Summary
One of the most important biological functions of macrophages is phagocytosis. By removing micro-organisms, apoptotic bodies, and foreign particles, macrophages phagocytosis is instrumental to several physiological and pathological processes. Binding to phagocytic receptors on macrophages triggers the production of calcium-mobilizing second messengers, such as 1,4,5-inositol-triphosphate and sphingosine-1-phosphate. Mobilization of calcium from lysosomes can contribute to spikes of [Ca2+], but the role of mitochondrial calcium uptake in phagocytosis is largely unknown. MCU is a major driver of mitochondrial calcium uptake, it has low affinity for calcium such that a high [Ca2+]c is required for a significant calcium entry into mitochondria. Mitochondria could uptake calcium when [Ca2+]c elevates, such as during phagocytosis, especially at microdomains of very high [Ca2+]c. Mitochondrial calcium uptake contributes to buffering [Ca2+]c, and regulates mitochondrial function to determine cellular metabolism and cell death processes
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