Time-dependent Amino Acid Crosstalk Shapes Macrophage Responses in Necrotizing Enterocolitis

In premature infants, protecting the healthy development of the intestine is a major clinical challenge that affects both the short-term and long-term growth of the baby. However, up to 10% of premature infants develop the life-threatening intestinal disease of necrotizing enterocolitis (NEC) that significantly damages not only the intestine but also leads to systemic inflammation and potential long-term complications in survivors.¹ Extensive investigations of NEC pathogenesis have advanced our understanding of the inflammatory and epithelial injury pathways and the recognition that breast milk confers significant protection against NEC.² This recognition has catalyzed intensive research into identifying the specific bioactive components in breast milk responsible for the protective effects, such as immunoglobulins, human milk oligosaccharides, and free fatty acids.³ In addition to these well-studied components, breast milk also contains free amino acids whose contributions to intestinal development and NEC pathogenesis is far less understood.

To address this knowledge gap, Han et al focused on the 2 most abundant free amino acids in breast milk, glutamine and glutamate, to determine their potential roles in modulating disease-relevant pathways in NEC. Although some prior studies have suggested that glutamine may be protective against NEC disease, the overall evidence has remained inconclusive. In the current study in Cellular and Molecular Gastroenterology and Hepatology, Han et al⁴ integrated a rodent model of NEC, targeted in vitro assays, and analysis of published single-cell RNA sequencing data to delineate how glutamine and glutamate influence NEC disease and, specifically, within macrophages of the neonatal intestine. The investigators examined the impact of these free amino acids both as prophylactic supplements prior to NEC induction and as therapeutic interventions following early disease onset. This approach addresses an important but often overlooked distinction in NEC research and, in doing so, yields particularly meaningful clinical insights alongside clear mechanistic findings.

In their study, Han et al demonstrated that the timing of amino acid supplementation is critical in its effects on NEC pathogenesis. In the rodent model of NEC, pretreatment with glutamine and glutamate conferred protection against NEC injury, whereas administration after the onset of damage did not provide therapeutic benefit. To investigate mechanisms of this timing effect, the authors focused on macrophages, as these immune cells exhibit high glutamine utilization and are implicated in NEC-associated inflammation, as confirmed by their analysis of a published single-cell RNA sequencing dataset.⁵ Both in vivo and in vitro studies demonstrated that prophylactic supplementation of glutamine and glutamate reduced macrophage’s proinflammatory cytokine expression, while post-onset supplementation either had no effect, or in many cases, increased inflammatory responses. These findings delineate the critical importance of timing in amino acid supplementation and the balance between protection and exacerbation based on the stage of disease onset.

In addition to these timing-dependent effects, Han et al also identified a shift in the glutamine metabolism pathways, specifically within macrophages. Inflammatory stimulus from either lipopolysaccharide or NEC disease increased glutaminase expression while reducing glutamate dehydrogenase 1 and glutamine synthase. Similarly, the corresponding metabolites produced by these enzymes also shifted, with an increase of succinate paired with a reduction in α-ketoglutarate. This expression pattern in the glutamine-associated enzymes and metabolites is indicative of a metabolic flux towards proinflammatory activity in macrophages. Prophylactic glutamine supplementation attenuated this proinflammatory metabolic programming in both lipopolysaccharide-stimulated bone marrow-derived macrophages and macrophages in NEC-injured ileum, although succinate supplementation administered concurrently with glutamine negated this protective effect of glutamine pretreatment by increasing the abundance of tumor necrosis factor-α-expressing macrophages in the NEC model. These findings reveal a critical metabolic axis that connects free amino acids to macrophage inflammatory activity. Specifically, they provide mechanistic insight into how free amino acids from either breast milk or targeted supplementation can shape intestinal immune responses and impact NEC pathogenesis.

These findings from Han et al expand our understanding of the metabolic fluxes in macrophages involved in NEC pathogenesis and illustrate the protective contributions of the free amino acids glutamine and glutamate naturally found in breast milk. These insights can also be utilized to develop and optimize clinical support for NEC prevention strategies. In particular, targeted and time-specific supplementation with glutamine may offer a prophylactic strategy for preterm infants at heightened risk of NEC. At the same time, the results emphasize the need for caution against administration after disease onset. This deliberate investigation into free amino acids and macrophage interactions, as well as interventional timing, provides a strong rationale for future studies aimed at validating these mechanisms in human NEC and designing carefully timed supplementation studies. These studies can help optimize feeding and fortification protocols that are targeted for healthy intestinal and immune maturation in vulnerable preterm infants. Overall, Han et al delineated a tractable metabolic axis at the intersection of free amino acid availability, neonatal immunity, and disease timing that can inform the development of more effective preventative approaches for NEC.