Response to Letter Regarding Article, “Cardiac pericytes acquire a fibrogenic phenotype and contribute to vascular maturation after myocardial infarction”

I greatly appreciate the very interesting comments by Dr Zhenqing Teng et al. on our manuscript investigating the role of pericytes in the infarcted heart¹. Two recently published studies¹,² combined lineage tracing experiments, genetic approaches for pericyte-specific targeting and single cell RNA-seq to study the fate and function of pericytes in repair, remodeling and fibrosis after myocardial infarction. The findings suggest both reparative and maladaptive functions of infarct pericytes. In the healing infarct, pericytes regulate vascular permeability and promote repair by coating infarct neovessels to generate a mature scar vasculature. These effects restrain post-infarction inflammation thus attenuating adverse remodeling. On the other hand, during the proliferative phase of infarct healing, infarct pericytes acquire a fibrogenic phenotype, expressing extracellular matrix proteins and fibroblast-activating growth factors. Lineage tracing experiments coupled with scRNA-seq showed that a subpopulation of hyperactive infarct fibroblasts (~4–5% of all PDGFRα+ fibroblasts) is derived from pericytes¹. The pro-fibrotic actions of the pericytes may contribute to the pathogenesis of post-infarction heart failure. Considering their diverse range of functions, both reparative and maladaptive, pericytes are challenging therapeutic targets. Teng et al. suggest that reprogramming of cardiac pericytes into induced cardiomyocytes may hold therapeutic promise. Although cellular reprogramming of mesenchymal cells to remuscularize the heart is a very exciting research direction, efficiency of currently available strategies is very low, and generation of functional myocardium remains a distant visionary goal. Thus, from a therapeutic perspective, it may be more practical to consider the reparative and maladaptive functions of pericytes when designing therapeutic approaches to attenuate remodeling with more conventional targets, such as Transforming Growth Factor (TGF)-β, or other cytokines and growth factors.

As Teng and co-workers point out, the in vivo findings from genetically targeted mice suggest that TGF-β may mediate both fibrosis and vascular maturation in the infarcted heart¹,². Our study used inducible pericyte-specific TGFBR2 KO mice, to demonstrate a central reparative role for TGF-β-induced activation of mural cells, mediated through stimulation of vascular maturation in the healing infarct¹. On the other hand, Quijada et al.² generated mice with pericyte-specific TGFBR1 loss (using the same Cspg4-CreER driver) and demonstrated fibrogenic actions of TGF-β on pericytes that may contribute to dysfunction. The seemingly conflicting findings may reflect the different TGF-β-mediated pathways targeted by each one of these approaches. Loss of the type 2 receptor TGFBR2 abrogates all effects of the 3 TGF-β isoforms, whereas TGFBR1/Activin-Like Kinase (ALK)5 deletion may preserve actions mediated through other type 1 receptors, such as ALK1. Dissection of the signaling cascades mediating the effects of TGF-β superfamily members in pericytes may be needed to identify strategies for selective targeting of TGF-β-induced fibrogenic pericyte activation.

Finally, Teng and colleagues discuss interesting future experiments to enrich our knowledge on the fate and mechanisms of activation of pericytes in injured and remodeling hearts. They suggest the use of a “pan-mesenchymal” Pdgfrb-CreER driver, coupled with single cell studies and spatial multiomic approaches, to better understand the patterns of differentiation of pericytes in the infarcted heart. Although we have considered using the inducible Pdgfrb-CreER driver, our early experiments aimed at validating pericyte-specific tools showed that this driver targets both mural cells and a large subpopulation of fibroblasts³, limiting the ability to detect pericyte to fibroblast conversion. However, we agree that spatial transcriptomic analysis to study the relation between topography and transcriptomic profile would be an extremely valuable tool to distinguish the fibrogenic matrix-producing interstitial pericytes from the mural cells that interact with the endothelium contributing to vascular maturation. We also greatly appreciate the suggestion regarding the use of human induced pluripotent stem cell-derived cardiac pericytes (iPSC-CP)⁴, which along with isolated human cardiac pericytes⁵ may provide valuable and translationally relevant in vitro models to dissect mechanisms of myocardial pericyte activation.