Mesenchymal Stem Cells Drive Cardiac Stem Cell Chemotaxis, Proliferation and Phenotype via CXCR4 and cKit signaling

Heart failure remains one of the greatest human health threats, accounting for nearly one third of all deaths within the United States and with global penetrance. ¹ Ischemic cardiomyopathy is a major contributor to HF with massive numbers of cardiomyocytes lost as a result of myocardial infarction (MI). Post MI, ongoing apoptosis is a significant contributor to heart failure progression regardless of etiology, exacerbating the loss of viable myocardium. With the discovery that cardiac regeneration can occur, cell therapy emerged as a novel and safe approach to cardiac repair and regeneration.

Historically, cardiac regeneration research has focused on delivering stem cells of mesenchymal and hematopoietic origin to the heart. Myocardial residence times of delivered cells have been shown to be discouragingly short, casting doubt on their regenerative potential in the heart. Excitement levels remain high because: 1.) Mesenchymal Stem Cells (MSCs) express beneficial paracrine factors 2.) Resident cardiac stem cells have cardiomyogenic potential.² Endogenous cKit⁺ resident cardiac stem cells (CSCs) have multiple lineage potential. 4.) MSC or CSC delivery to the failing heart results in improved cardiac function, angiogenesis and detection of CSCs with cardiomyocyte phenotypes.³ 5.) MSCs communicate with CSCs leading to reduced infarct size, and improved cardiac performance.^4-6 6.) Paracrine factors are key effectors of endogenous CSC responses in the failing heart leading to cardiac repair. This enthusiasm has been tempered by a lack of mechanistic insights regarding the roles of both MSC and CSC roles in improving outcomes for Heart Failure (HF) patients. Also, some have documented minimal CSC cardiomyogenesis, shedding some doubt that they mature into functioning cardiomyocytes in the mammalian heart.⁷ Thus, a more complete mechanistic understanding of MSC signaling in heart and their effects on endogenous CSCs in the failing heart remain unmet needs.

Hatzistergos et al. now show that at least two key signaling cascades are involved in MSC induced CSC chemotaxis, proliferation and differentiation to cardiomyocytes.⁸ This report has several key messages. Explants from murine neonatal hearts contain CSCs that are cardiomyogenic at very low frequencies. CSCs were not found to have an endothelial phenotype or differentiate into endothelium. Co-cultures of cardiac explants and MSCs induced CSC migration from the explants, proliferation and differentiation via SDF-1/CXCR4 and SCF/cKit signaling. These results are not limited to neonatal CSCs since iPSC derived CSCs behave similarly. Clinical significance was demonstrated using cardiosphere CSCs from dilated cardiomyopathy patients.

CSC,⁹ MSC¹⁰ and dual CSC/MSC cell therapies⁴ have been investigated in clinical and pre-clinical models of heart failure. Interestingly, clinical studies have shown reduced scar size and improved cardiac function. MSC delivery also results in improved cardiac perfusion in ischemic cardiomyopathy. Additionally, increased myocardial mitotic activity was reported after dual MSC and CSC cell therapy to a pig model of HF. These promising results could be at least partially due to upstream CXCR4 and cKit signaling and are likely the focus of future pre-clinical investigations. Since Hatzistergos et al. did not detect significant CSC differentiation to endothelium in their explants, other hMSC driven increases in cardiac perfusion, potentially by additional signaling cascades still need to be explored in detail. In the setting of ischemic cardiomyopathy, gene delivery of SDF-1α resulted in recruitment of CD133⁺ cells to the myocardium, angiogenesis, and led to improved cardiac function.¹¹ In tissue culture, cardiosphere derived CSCs responded to hypoxia by up-regulating their CXCR4 expression levels and increasing their migration¹². When these hypoxia-induced cells were delivered intravenously to ischemic mouse hearts, they homed to the peri-infarct area in the myocardium, and improved outcomes. Thus, there is strong support for MSC driven homing to the infarct site, proliferation and differentiation via both the CXCR4 and cKit receptors in the setting of ischemic HF. The subtle differences found between study conclusions are likely due to experimental design differences and crosstalk between cKit and CXCR4.¹³ The benefits of SDF1α have also been documented in a clinical trial,¹⁴ showing improvement in quality of life.

Adenoviral delivery of SCF (Ad.SCF) in the setting of ischemic cardiomyopathy resulted in improved outcomes.^{15, 16} Cellular and molecular endpoints were consistent with CSC homing, proliferation, cardiomyocyte cell cycling and reduced apoptosis in rats¹⁷ and pigs.¹⁵ In pigs, increased angiogenesis was documented, suggesting that the improved cardiac function and better survival resulted at least in part due to improved cardiac perfusion in the peri-infarct regions. The greater densities of neovasculature in pigs were likely due to chemotaxis and proliferation of CSCs followed by downstream paracrine signaling to endothelial progenitors, though the involvement of CXCR4 mediated repair in these HF studies is not known. In these cases, cKit activation appears to result in greater downstream signaling than in the murine explant system.

Hatzistergos et al. have shown that MSCs impart profound effects on CSCs.⁸ This report is a gateway to understanding how MSC delivery to the failing heart improve outcomes. Their mechanistic investigation reveals that CSC chemotaxis and differentiation to cardiomyocytes is regulated by MSC expressed SDF-1α while proliferation can be induced by MSC expressed SCF. This work explains the results of several earlier reports and brings us to a critical milestone in understanding some of the paracrine effects that MSCs impart on CSCs. More detailed mechanistic information is needed to understand cross-talk between CXCR4 and cKit on SDF-1α or SCF ligand binding to their respective targets.^{13, 17} A more precise understanding of mechanisms underlying improved cardiac perfusion from MSC delivery will enable translation of novel therapeutic approaches to cell delivery, gene delivery and pharmacotherapies for HF patients.