Macrophages Lacking TSC2 have mTORC1-dependent GPNMB Augmentation Ameliorating Cardiac Ischemia-Reperfusion Injury

Macrophages (MΦ) modulate both myocardial inflammatory and reparative phases following ischemia-reperfusion (I/R) injury. The mechanistic target of rapamycin (mTOR) is thought to play an important role in MΦ phenotype and functionality, but studies report conflicting net influences suggesting dependence on disease context and downstream signaling. Here, we tested the impact of MΦ with constitutive mTORC1 activation induced by targeted deletion of tuberous sclerosis complex 2 (TSC2) on cardiac responses to I/R injury.

Myeloid TSC2 depleted (MΦ^TSC2-/-) mice were generated by crossing Lys2^Cre x TSC2^flx/flx. Bone-marrow derived MΦ^TSC2-/- vs control MΦ had basal increased mTORC1 and reduced mTORC2 activity. MΦ^TSC2-/- were differentially responsive to stimulation by lipopolysaccharide/IFN-γ or IL-4 in vitro, and all disparities were prevented by rapamycin confirming the model. In vivo, MΦ^TSC2-/- mice were strongly protected against I/R injury, with minimal change in ejection fraction, less LV dilation, hypertrophy, lung edema, or activation of stress/pro fibrotic genes. Mice pre-treated with anti-LY6G Ab to deplete neutrophils were still similarly protected, suggesting that the impact was primarily related to MΦ. MΦ^TSC2-/- mice had less myocardial pro-inflammatory macrophages (CCR2⁺MHC-II^hi), LY6C⁺ monocytes, neutrophils, and CD8⁺ T cells 5 days post-I/R, and fewer CCR2⁺ but more CCR2^- MΦ 2 weeks post I/R. Both MΦ^TSC2-/- in vitro and in vivo post I/R phenotypes were converted to WT by rapamycin, supporting mTORC1 dependence. Lastly, synthesis of glycoprotein nonmetastatic melanoma protein B (GPNMB), a principally MΦ anti-inflammatory secreted protein protective against myocardial infarction was enhanced in MΦ^TSC2-/- macrophages and hearts following I/R in an mTORC1 dependent manner.

Constitutive macrophage-specific mTORC1 activation via TSC2 deletion reduces pro-inflammatory cell infiltration, increases GPNMB protein expression and preserves heart function following I/R injury. Rapamycin eliminates these effects. These results identify a cardioprotective mTORC1-GPNMB signaling nexus in MΦ in vivo.