Sustained Generation of Nitric Oxide and Control of Mycobacterial Infection Requires Argininosuccinate Synthase 1

SummaryNitric oxide (NO) defends against intracellular pathogens, but its synthesis must be regulated due to cell and tissue toxicity. During infection, macrophages import extracellular arginine to synthesize NO, generating the byproduct citrulline. Accumulated intracellular citrulline is thought to fuel arginine synthesis catalyzed by argininosuccinate synthase (Ass1) and argininosuccinate lyase (Asl), which would lead to abundant NO production. Instead, we find that citrulline is exported from macrophages during early stages of NO production with <2% retained for recycling via the Ass1-Asl pathway. Later, extracellular arginine is depleted, and Ass1 expression allows macrophages to synthesize arginine from imported citrulline to sustain NO output. Ass1-deficient macrophages fail to salvage citrulline in arginine-scarce conditions, leading to their inability to control mycobacteria infection. Thus, extracellular arginine fuels rapid NO production in activated macrophages, and citrulline recycling via Ass1 and Asl is a fail-safe system that sustains optimum NO production.

Graphical AbstractFigure optionsDownload high-quality image (355 K)Download as PowerPoint slideHighlights
► Citrulline, a byproduct of nitric oxide (NO) synthesis, is exported from macrophages
► Arginine is synthesized from imported citrulline to fuel NO synthesis in late infection
► Macrophages lacking Ass1 fail to synthesize arginine from citrulline
► Ass1 is required for optimal control of M. bovis BCG and M. tuberculosis