Macrophage SLC9A1 Links Endocytic Trafficking to Innate Immune Activation in Myocardial Injury

Excessive innate immune activation drives adverse remodeling after myocardial infarction (MI), yet the upstream mechanisms by which macrophages sense ischemic danger signals remain poorly defined. Here we tested whether macropinocytosis functions as a mediator of post-ischemic inflammation and whether the Na⁺/H⁺ exchanger SLC9A1 links membrane ion transport to innate immune activation in the injured heart. Macropinocytosis was robustly activated in infarct-associated macrophages, which are the predominant cell type with the macropinocytotic activity in the injured heart. Pharmacologic inhibition of macropinocytosis with 5-(N-ethyl-N-isopropyl)amiloride (EIPA) improved cardiac function and attenuated post-MI remodeling. EIPA also attenuated cardiac inflammatory responses induced by systemic lipopolysaccharide and Poly(I:C). To define macrophage-intrinsic mechanisms, we generated monocyte- and monocyte-derived macrophage-specific <i>Slc9a1</i> knockout mice. Genetic deletion of <i>Slc9a1</i> recapitulated the cardioprotective effects of EIPA and markedly suppressed interferon-stimulated gene programs in infarct-associated macrophages, as revealed by single-cell RNA sequencing. Mechanistically, SLC9A1 promoted endocytic uptake of Poly(I:C) acid and enhanced endosome-dependent inflammatory signaling. Together, these findings identify macrophage macropinocytosis as a regulator of innate immune activation after MI and reveal SLC9A1 as a previously unrecognized link between membrane ion transport and inflammatory signaling in the injured heart. Targeting SLC9A1-dependent membrane trafficking pathways may therefore represent a strategy to limit maladaptive inflammation in ischemic heart disease.