Comparative bulk and single-cell transcriptome analyses revealed distinct functional polarization of cardiac macrophages in regenerative and non-regenerative mouse hearts
Authors: Yu H.C , Liou K.Y , Wu P.C , Chang C.F , Lai S.L , Yang K.C 
Published: November 2020

Abstract

Background
Heart failure is a major cause of morbidity and mortality, in part owing to the inability of the human heart to replenish lost cardiomyocytes following cardiac injury. Recent studies have demonstrated that neonatal human and mouse heart can fully regenerate in response to injury. This regenerative capacity, however, is transient and age-dependent. The regenerative capacity of neonatal mouse heart, for example, declines sharply by 7 days of age. It has been demonstrated that tissue-resident macrophages in neonatal hearts are required for cardiac repair and recovery from injuries by promoting angiogenesis and cardiomyocyte proliferation. The cellular and molecular determinants underlying the reparative/pro-regenerative properties of neonatal cardiac macrophages, however, remain elusive.

Purpose
To reciprocally characterize macrophage heterogeneity and functionality that are critical for myocardial repair and regeneration following myocardial infarction (MI) injury in neonatal (regenerative) and adult (non-regenerative) mouse hearts.

Methods and results
Cardiac tissue-sorted macrophages (CD45+CD11b+CD64+) from neonatal (P1–3) and adult (8 weeks) mouse heart 10 days following MI (by permanent coronary artery ligation) were subjected to bulk and single-cell RNA sequencing (scRNA-Seq) analyses. In response to MI injury, CCR2+ monocyte-derived macrophages predominate in adult, whereas CCR2- resident macrophages expand markedly in neonatal mouse hearts. Comparing to sham-operated controls, bulk RNA-Seq revealed that the expression levels of a total of 482 and 614 mRNAs were significantly (absolute fold change &gt;2, P&lt;0.05) altered in adult and neonatal cardiac macrophages following MI, respectively. Pathway analyses showed that genes involved in olfactory function and Wnt-signaling were dysregulated in adult, whereas transcripts involved in oxidative phosphorylation and interferon-g (IFN-g) pathway were upregulated in neonatal cardiac macrophages after MI. scRNA-Seq analysis identified 11 distinct cardiac macrophage clusters in adult mouse heart following MI, several of which showed strong upregulation in inflammatory mediators (Ly6c2, Clec4b1, Cxcl3 and Lilra5) and extracellular matrix protein genes (Fn1, and Dcn), suggesting macrophage polarization towards a pro-inflammatory and fibrogenic phenotype in post-injury adult heart.

Conclusion
As distinct macrophage populations mentioned in current study, our results revealed divergent transcriptionl signatures of cardiac macrophages in post-MI neonatal and adult mouse hearts. While Wnt signaling, inflammation and fibrogenic activities are increased in adult cardiac macrophages, oxidative phosphorylation and IFN-g signaling are markedly activated in neonatal cardiac macrophages in response to injury. Phenotypic switching toward enhanced oxidative phosphorylation and IFN-g signaling in cardiac macrophages, therefore, could be the key to develop novel therapeutics to promote cardiac regeneration.

Funding Acknowledgement
Type of funding source: None