Finding Expandable Induced Cardiovascular Progenitor Cells

Abstract

Cardiovascular progenitor cells (CPCs) are a promising cell source for cardiac regenerative therapy owing to their ability for self-renewal and differentiation into various cardiovascular cell types beneficial for myocardial repair: cardiomyocytes, smooth muscle cells, and endothelial cells. Previous evaluations of exogenously derived CPCs have focused mainly on their capacity for tri-lineage differentiation rather than self-renewal, owing to the lack of an effective protocol to maintain and expand CPCs long-term in culture. In a recent issue of Cell Stem Cell, two groups of investigators independently reported their success in isolating, maintaining, and expanding mouse CPCs in culture for greater than 18 to 20 passages (10¹⁰- to 10¹⁵-fold expansion), with faithful preservation of progenitor phenotype and ability for tri-lineage-restricted differentiation in both cell culture and mouse models of myocardial infarction. This Commentary will discuss the unique findings of these two studies, highlight the strengths and weaknesses of each CPC derivation/expansion technique, and propose additional steps necessary to accelerate their clinical translation.

Figure 1. Generation of expandable induced cardiovascular progenitor cells

Two different approaches for isolating expandable induced cardiovascular progenitor cells (iCPCs) are presented. In cell activation and signal-derived (CASD) reprogramming (red solid arrow), somatic cells (e.g., fibroblasts) are transiently exposed to Yamanaka factors (Oct4, Sox2, Klf4, and c-Myc; abbreviated as OSKM) followed by cardiac specification towards iCPCs using JAK inhibitor (Jl1) and GSK3 inhibitor (CHIR99021). Unlike conventional nuclear reprogramming for generation of induced pluripotent stem cells (iPSCs) (green arrow), the derivation of iCPCs via CASD reprogramming does not involve a transition through the pluripotent state, which is prevented by JAK inhibition. The iCPCs generated, identified by Nkx2-5⁺ positivity, can be expanded in culture using a cocktail of small molecules termed BACS (BMP4, Activin A, CHIR99021, SU5402) (red dotted arrow). In another approach, iCPCs can be derived by direct reprogramming using overexpression of 5 transcriptional factors, including Mesp1, Tbx5, Gata4, Nkx2-5, and Baf60c (MTGNB) (blue solid arrow). The iCPCs generated, identified by Flk-1⁺/PdgfR-α⁺ positivity, can be expanded using a combination of LIF (JAK/STAT activator) and BIO (Wnt activator) (blue dotted arrow). Note that iCPCs also form as intermediates during the differentiation of iPSCs/ESCs towards distinct cardiovascular cell types (paths indicated by grey arrows): cardiomyocytes (CMs), endothelial cells (ECs), and smooth muscle cells (SMCs).