Abstract P1004: Macrophage Apoptotic Cell Receptor Mertk Triggers The Synthesis Of Arachidonic Acid Derivatives To Promote Cardiac Regeneration

Abstract

Introduction: Macrophages (MΦs) are indispensable for regeneration of the mammalian neonatal heart. However, the underlying mechanisms by which neonatal MΦs sense the injured cardiac milieu and promote regeneration remain obscure. Utilizing single-cell transcriptomics, we previously observed a regenerative neonatal cardiac MΦ population that distinctly upregulates MerTK, an apoptotic cell receptor, compared to adult cardiac MΦs. Objective: We hypothesized that MerTK is required to program neonatal cardiac MΦs to secrete specialized pro-“regenerative” lipids to induce cardiomyocyte proliferation leading to heart regeneration. Methods: Postnatal day 1 and adult mice were subjected to cardiac injury via ligation of the left anterior descending artery. Single-cell transcriptomics, lipid-omics, and molecular approaches were utilized to determine the mechanisms by which neonatal MΦs regulate cardiac regeneration. Results: To determine the necessity of MΦ MerTK for cardiac regeneration, we employed a myeloid-specific knockout of MerTK ( mMertk -/- ) which failed to regain homeostatic cardiac function. We next performed single-cell transcriptomics on Mertk +/+ and Mertk -/- neonates to determine how MerTK activation reprograms neonatal MΦs for regeneration. Mertk +/+ MΦs significantly upregulated genes associated with arachidonic acid metabolism. Hypothesizing that arachidonic acid derivatives may be key signaling molecules for regeneration, we performed a targeted lipid-omics screen which revealed an increase in Thromboxane B 2 (TXB 2 ) in regenerating hearts but not within non-regenerative Mertk -/- hearts. Furthermore, addition of a TXA 2 agonist to neonatal cardiomyocytes in vitro significantly increases their proliferation by regulating the YAP/TAZ signaling. Alternatively, inhibition of thromboxane synthase in neonatal MΦs or thromboxane receptor on cardiomyocytes by small molecule inhibitors significantly abrogated the increase in cardiomyocyte proliferation by efferocytic Mertk +/+ neonatal MΦs. Conclusions: Altogether, our data suggest that MerTK programs neonatal cardiac MΦs to secrete TXA 2 to induce cardiomyocyte proliferation via the HIPPO-YAP/TAZ pathway prompting neonatal cardiac regeneration.