Differential Requirements for l-Citrulline and l-Arginine during Antimycobacterial Macrophage Activity

Abstract

Microbicidal NO production is reliant on inducible NO synthase–mediated l-arginine metabolism in macrophages (MΦs). However, l-arginine supply can be restricted by arginase activity, resulting in inefficient NO output and inhibition of antimicrobial MΦ function. MΦs circumvent this by converting l-citrulline to l-arginine, thereby resupplying substrate for NO production. In this article, we define the metabolic signature of mycobacteria-infected murine MΦs supplied l-arginine, l-citrulline, or both amino acids. Using liquid chromatography–tandem mass spectrometry, we determined that l-arginine synthesized from l-citrulline was less effective as a substrate for arginase-mediated l-ornithine production compared with l-arginine directly imported from the extracellular milieu. Following Mycobacterium bovis bacillus Calmette–Guérin infection and costimulation with IFN-γ, we observed that MΦ arginase activity did not inhibit production of NO derived from l-citrulline, contrary to NO inhibition witnessed when MΦs were cultured in l-arginine. Furthermore, we found that arginase-expressing MΦs preferred l-citrulline over l-arginine for the promotion of antimycobacterial activity. We expect that defining the consequences of l-citrulline metabolism in MΦs will provide novel approaches for enhancing immunity, especially in the context of mycobacterial disease.

Amino acid auxotrophy as a system of immunological control nodes

“…Consistent with this idea, mice lacking ASS1 in bone marrow-derived macrophages were lethally infected with Mtb compared to cognate control mice, arguing for an essential role of the ASS1-ASL-mediated arginine regeneration system in infection immunity 50 . Recent experiments manipulating the amounts of arginine and citrulline in the media revealed when M1 macrophages produce NO from citrulline using the arginine regeneration pathway, Arg1 is no longer capable of blocking NO production 60 . These and other data argue activated macrophages compartmentalize arginine metabolism in some way, possibly by sub-cellular partitioning of the key enzymes.…”

Macrophage Function in the Pathogenesis of Non-alcoholic Fatty Liver Disease: The Mac Attack

“…As such, studies determining the contribution not only of L-ARG utilization, but also of L-ARG synthesis during NAFLD are warranted. The necessity of L-ARG synthesis within macrophages has recently been described during infection, suggesting extracellular L-ARG is not available in sufficient concentrations to drive effective macrophage function (135,146,147). Accounting for the considerable influx of inflammatory macrophages in NAFLD, future studies aimed at addressing macrophage-specific modulation of L-ARG metabolism with existing molecular tools (e.g., Arg1 flox , Asl flox , Nos2-deficient) (148-150) will be necessary to dissect how various macrophage populations manipulate the liver microenvironment and NAFLD progression.…”

l-Arginine Synthesis from l-Citrulline in Myeloid Cells Drives Host Defense against Mycobacteria In Vivo

“…Antimycobacterial NO production and T cell activity can be fueled by a related amino acid, L-citrulline (L-CIT) (17)(18)(19)(20). L-CIT can be acquired from the diet or during metabolic processing of L-ARG (via NO synthases) or L-ORN (via ornithine carbamoyl transferase) (21).…”

“…Also, it can be used to synthesize L-ARG, entailing the sequential activity of two cytosolic enzymes: argininosuccinate synthase (Ass1) and argininosuccinate lyase (Asl) (21). We have recently uncovered an immune-protective role of this pathway during infection with M. tuberculosis and Mycobacterium bovis bacillus Calmette-Guérin (BCG), in which L-ARG synthesized from L-CIT bypasses arginase-mediated inhibition of NO production and benefits host defense against mycobacterial infection (19,20).…”