Cardiac Pericytes Acquire a Fibrogenic Phenotype and Contribute to Vascular Maturation After Myocardial Infarction

Abstract

BACKGROUND: Pericytes have been implicated in tissue repair, remodeling, and fibrosis. Although the mammalian heart contains abundant pericytes, their fate and involvement in myocardial disease remains unknown. METHODS: We used NG2 Dsred ;PDGFRα EGFP pericyte:fibroblast dual reporter mice and inducible NG2 CreER mice to study the fate and phenotypic modulation of pericytes in myocardial infarction. The transcriptomic profile of pericyte-derived cells was studied using polymerase chain reaction arrays and single-cell RNA sequencing. The role of transforming growth factor–β (TGF-β) signaling in regulation of pericyte phenotype was investigated in vivo using pericyte-specific TGF-β receptor 2 knockout mice and in vitro using cultured human placental pericytes. RESULTS: In normal hearts, neuron/glial antigen 2 (NG2) and platelet-derived growth factor receptor α (PDGFRα) identified distinct nonoverlapping populations of pericytes and fibroblasts, respectively. After infarction, a population of cells expressing both pericyte and fibroblast markers emerged. Lineage tracing demonstrated that in the infarcted region, a subpopulation of pericytes exhibited transient expression of fibroblast markers. Pericyte-derived cells accounted for ~4% of PDGFRα+ infarct fibroblasts during the proliferative phase of repair. Pericyte-derived fibroblasts were overactive, expressing higher levels of extracellular matrix genes, integrins, matricellular proteins, and growth factors, when compared with fibroblasts from other cellular sources. Another subset of pericytes contributed to infarct angiogenesis by forming a mural cell coat, stabilizing infarct neovessels. Single-cell RNA sequencing showed that NG2 lineage cells diversify after infarction and exhibit increased expression of matrix genes, and a cluster with high expression of fibroblast identity markers emerges. Trajectory analysis suggested that diversification of infarct pericytes may be driven by proliferating cells. In vitro and in vivo studies identified TGF-β as a potentially causative mediator in fibrogenic activation of infarct pericytes. However, pericyte-specific TGF-β receptor 2 disruption had no significant effects on infarct myofibroblast infiltration and collagen deposition. Pericyte-specific TGF-β signaling was involved in vascular maturation, mediating formation of a mural cell coat investing infarct neovessels and protecting from dilative remodeling. CONCLUSIONS: In the healing infarct, cardiac pericytes upregulate expression of fibrosis-associated genes, exhibiting matrix-synthetic and matrix-remodeling profiles. A fraction of infarct pericytes exhibits expression of fibroblast identity markers. Pericyte-specific TGF-β signaling plays a central role in maturation of the infarct vasculature by protecting from adverse dilative remodeling, but it does not modulate fibrotic remodeling.

Cardiac Pericyte Diversity in Infarct Remodeling: Not Just Vascular Support Cells?

“…6 However, no bona fide pericyte-derived fibroblasts, ie, with loss of mural cell markers and upregulation of fibroblast lineage markers, were identified. Here, Alex et al 11 report a similar process of pericyte activation, and uncover that at day 7 after MI, a subset of pericytes coexpressing fibroblast marker PDGFRα EGFP expressed some extracellular matrix genes to a greater extent than activated fibroblasts. Also, 16.6% of infarct myofibroblasts, defined as elongated α-SMA + cells located outside the vascular wall, were NG2 CreERlineage-traced.…”

“…5 However, in mice, genetic ablation of Tgfbr2 in the NG2 lineage did not affect pericyte-to-myofibroblast transition or infarct scar size. Instead, Alex et al 11 observed reduced numbers of pericytes encircled by forming arterioles. This was associated with reduced coverage of vessels by vascular support cells, vessel dilation, increased infiltration of macrophages, and increased left-ventricular enddiastolic volume 1 week after infarction.…”

“…In the future, it will be important to selectively target profibrotic and proangiogenic properties of pericytes to improve remodeling after MI. Furthermore, pericyte heterogeneity described by Alex et al 11 highlights the possibility of additional roles for pericytes in infarct resolution that may also provide future therapeutic targets in the setting of MI.…”

“…Markers distinguishing these cell types include NG2 and PDGFRα, which label pericytes and fibroblasts, respectively, as corroborated by recent single cell sequencing-based experiments. 10 In this issue of Circulation, Alex et al 11 used dual NG2 DsRed /PDGFRα EGFP reporters and NG2 CreER lineage tracing to perform histological and fluorescence-activated cell sorting analyses to investigate fates and functions of pericytes during the inflammatory (3 days), proliferative (7 days), and maturation phases (28 days) of post-MI remodeling. In heathy myocardium, NG2 DsRed+ pericytes were negative for fibroblast marker PDGFRα EGFP and associated with small vessels (Figure [A]).…”

“…Activation of a fibrogenic program in pericytes raised the question of their fate during postinfarct remodeling. Alex et al 11 performed inducible NG2 CreER -based lineage tracing, revealing that 16.6% of infarct "myofibroblasts, " defined as αSMA + cells outside vascular walls, were derived from pericytes (Figure [C]). Neovascularization of the infarct area is a key determinant of postinfarct remodeling.…”