Migration of Resident Cardiac Stem Cells in Myocardial Infarction

Authors

  • Simon X. Liang,

    Corresponding author
    1. Department of Biochemistry and Molecular Biology, College of Basic Medical Sciences, Liaoning Medical University, Jinzhou City, Liaoning 121001, People's Republic of China
    • Department of Biochemistry and Molecular Biology, College of Basic Medical, Sciences, Liaoning Medical University, Jinzhou City, Liaoning 121001, P. R. China
    Search for more papers by this author
    • Fax: 86-461-4673008

  • William D. Phillips

    1. Physiology and Bosch Institute, Department of Physiology, University of Sydney, NSW 2006, Australia
    Search for more papers by this author

Abstract

Ischemic heart disease is a major cause of morbidity and mortality worldwide. Stem cell-based therapy, which aims to restore cardiac structure and function by regeneration of functional myocardium, has recently been proposed as a novel alternative treatment modality. Resident cardiac stem cells (CSCs) in adult hearts are a key cell type under investigation. CSCs have been shown to be able to repair damaged myocardium and improve myocardial function in both human and animal studies. This approach relies not only on the proliferation of the CSCs, but also upon their migration to the site of injury within the heart. Here, we briefly review reported CSC populations and discuss signaling factors and pathways required for the migration of CSCs. Anat Rec, 2013. © 2012 Wiley Periodicals, Inc.

The prevailing view of the heart as a terminally differentiated organ has been challenged by the discovery of resident cardiac stem cells (CSCs), which may bring the treatment of cardiac diseases into a new era. Post myocardial infarction (MI) heart failure is caused by the loss of cardiomyocytes due to ischemic necrosis and apoptosis. Thus, repairing or regenerating lost myocardium by stem cell therapies is highly appealing. In recent years various subsets of resident CSCs that are capable of self-renewal, proliferation, and differentiation into cardiomyocytes, endothelial and smooth muscle cells have been identified (Hierlihy et al., 2002; Beltrami et al., 2003; Oh et al., 2003; Messina et al., 2004; Eisenberg et al., 2004; Di Felice et al., 2009). CSCs have been shown to be able to migrate to the site of injury within the heart and participate in repairing damaged myocardium, leading to improved cardiac function following MI in different experimental MI models (Beltrami et al., 2003; Messina et al., 2004; Oh et al., 2004; Linke et al., 2005). The migration of CSCs to the infarct site is regulated by several signaling factors and pathways that are not yet fully understood (Urbanek et al., 2005; Kuang et al., 2008; Tang et al., 2009). In this review, we will briefly discuss the role of CSCs in cardiomyocyte homeostasis, myocardial repair and regeneration, and provide an overview of the signaling factors and pathways involved in CSC migration. These same pathways may contribute to the proliferation and survival of CSCs following MI.

RESIDENT CSCs

SCA1+/CD31 Cardiac SP Cells

Side-population (SP) cells, defined by their ability to actively efflux Hoechst 33342 dye, were first identified in bone marrow (Goodell et al., 1996). This unique property is mediated through the action of the ATP-binding cassette transporter proteins ABCG2 (Zhou et al., 2001) and Abcb1/Mdr1 (Pfister et al., 2008) expressed on the cell surface. SP cells with stem cell-like properties in adult hearts were first reported by Hierlihy et al. (2002). Since then, several groups, including ours, have confirmed the presence of such stem/progenitor cell populations in adult hearts (Martin et al., 2004; Pfister et al., 2005; Oyama et al., 2007; Liang et al., 2010).

On the basis of the expression of stem cell antigen 1 (SCA1) and CD31, SP cells from the mouse heart can be divided into two subpopulations: SCA1+/CD31 and SCA1+/CD31+ (Pfister et al., 2005; Liang et al., 2010; Liang et al., 2011). Pfister et al. (2005) demonstrated that only SCA1+/CD31 cardiac SP (CSP) cells were capable of differentiating into functional cardiomyocytes in vitro. We recently extended these findings, showing that following MI in mice SCA1+/CD31 CSP cells migrated from nonischemic myocardium into the infarcted area where they differentiated into both cardiomyocyte- and endothelial-like cells (Liang et al., 2010). Our work further demonstrated that the second sub-population: SCA1+/CD31+ CSP cells function as cardiac endothelial progenitor cells in mice. The SCA1+/CD31+ cells also migrated from nonischemic myocardium into the ischemic region where they differentiated into mature endothelial cells, forming a tube-like vascular structure following MI (Liang et al., 2011). SCA1+/CD31 and SCA1+/CD31+ CSP both up-regulated expression of C-X-C chemokine receptor type 4 (CXCR4) following MI. CXCR4 is a receptor for the chemotactic cytokine known as stromal-derived-factor-1 (SDF-1) (Liang et al., 2010; Liang et al., 2011). Similar findings were demonstrated in rats by Oyama et al. (2007).

The precise role of ABCG2 in CSP cells remains uncertain but seems to extend well beyond traditional functions of an ATP-binding cassette transporter. Recent studies indicate that ABCG2 is important for cytoprotection and for regulating the function and homeostasis of CSP cells. Using a loss- and gain-of-function strategy, Pfister et al. (2008) demonstrated that adult CSP cells isolated from ABCG2 knockout mice exhibited a blunted proliferation capacity, leading to increased cell death. In contrast, overexpression of ABCG2 significantly enhanced CSP cell proliferation but limited their capacity for cardiomyogenic differentiation. These results suggest that ABCG2 somehow serves to regulate adult CSP cell fate, function, and survival. Evidence from hematopoietic stem cells supports the notion that ABCG2 expression promotes cell survival. Krishnamurthy et al. (2004) have shown that the expression of ABCG2 can be upregulated by the hypoxia-inducible factor 1α (HIF-1α). Increased ABCG2 expression, in turn, facilitates the survival of bone marrow SP cells under hypoxic challenge. Similarly, Martin et al. (2008) demonstrated that resident ABCG2-expressing CSP cells substantially increase in number following myocardial cryoinjury in mice. Moreover, the increase of CSP cell numbers was accompanied by increased ABCG2 transcript expression in individual CSP cells, and the upregulation of ABCG2 expression is directly mediated by HIF-2α, another member of the hypoxia inducible transcriptional factor family. Importantly, these authors reveal that ABCG2 is able to protect CSP cells and enhance cell survival in response to hypoxic/ischemic injury, suggesting that ABCG2 plays functional roles in cell protection and survival. Taken together, these studies suggest that the expression of ABCG2 is upregulated by activation of hypoxia-HIFs interaction. Importantly, CSP cells expressing ABCG2 are able to survive, proliferate, and migrate in response to hypoxic/ischemic conditions. This knowledge may provide the basis for future therapeutic strategies, such as ischemic/hypoxic preconditioning, to enhance cardiac stem/progenitor cells survival, proliferation, differentiation, and migration.

C-kit+ CSCs

In 2003 Beltrami et al. (2003) characterized stem cells from adult hearts that expressed the cytokine receptor c-kit (c-kit+) and were negative for lineage markers (Lin). These c-kit+/Lin cells were able to undergo long term self-renewal, and differentiate into cardiomyocytes, endothelial and smooth muscle cells in vitro. When transplanted into mouse hearts after MI, c-kit+ CSCs retained the capacity for differentiation, facilitated a reduction of infarct size, and improved cardiac function. These cells were capable of migrating into the infarcted area from surrounding nonischemic myocardium. This process involves the SCF/c-kit and HGF/c-met pathways (Linke et al., 2005; Urbanek et al., 2005) discussed below. Bearzi et al. (2007) recently reported that human c-kit+ CSCs isolated from small samples of myocardium were able to expand, and differentiate into cardiomyocytes, endothelial and smooth muscle cells in vitro. A phase 1 clinical trial of intracoronary infusion of human c-kit+ CSCs in patients with post-infarction left ventricular dysfunction significantly reduced infarct size and improved left ventricular ejection fraction (LVEF) at 4 months (Bolli et al., 2011).

SCA1+ CSCs

Oh et al. (2003) were the first to identify a population of resident cardiac progenitor cells in adult mouse hearts based on their expression of SCA1. These SCA1+ cells are capable of differentiating into cardiomyocytes in vitro in the presence of 5-azacytidine (Oh et al., 2003) or oxytocin (Matsuura et al., 2004). Given to mice intravenously, SCA1+ cells that were positive for CXCR4 migrated to the injured myocardium and differentiated into cardiomyocytes in a mouse MI model (Oh et al., 2003, 2004). Further studies have shown that cells positive for SCA1 and Wilm's tumor 1 (Wt1) isolated from mouse heart are able to give rise to de novo cardiomyocytes that structurally and functionally integrate with resident muscle after myocardial injury (Smart et al., 2011). Chong et al. (2011) have recently demonstrated that a population of SCA1+ and platelet-derived growth factor receptor alpha (PDGFRα) positive cells derive from the proepicardium of mouse heart has capacity for clonogenic propagation, long-term in vitro growth, and multilineage differentiation both in vitro and in vivo.

Cardiosphere-Derived Cells

In 2004, small, self-adherent cell clusters were isolated from both murine and human hearts. Using a specialized culture method explants gave rise to spherical multicellular structures. These cells were defined as cardiosphere-derived cells (CDCs), another source of CSCs from adult heart (Messina et al., 2004). Cardiosphere-derived cells are a heterogeneous population, which are comprised of c-kit+, CD31+, and CD34+ cells (Davis et al., 2009). They are clonogenic and capable of long-term self-renewal. Furthermore, they were able to differentiate into cardiomyocytes, endothelial and smooth muscle cells both in vitro and in vivo (Messina et al., 2004; Smith et al., 2007). These cells highly expressed CXCR4 under hypoxic preconditioning (HPC), and were able to migrate to injured myocardium after intravenous injection (Tang et al., 2009). A phase 1 clinical trial of CDCs treatment has been recently carried out in patients 2–4 weeks after acute myocardial infarction (Makkar et al., 2012). Treatment with CDCs was reported to significantly reduce infarct size and increase viable heart mass at 6 months, but without significant improvement in LVEF.

Isl-1+ CSCs

Cardiac stem/progenitor cells expressing LIM homeodomain transcriptional factor Islet-1 (Isl-1+) were recently identified by Laugwitz and colleagues (Laugwitz et al., 2005). These Isl-1+ cells were isolated from very young animals, do not express c-kit but harbor cardiomyogenic potential both in vitro and in vivo. After 4 weeks of age, the Isl-1+ cardiac cells lost the capacity for differentiation (Weinberger et al., 2012). Thus, Isl-1+ cells are a population of cells with cardiac progenitor potential, but only in young hearts.

Several different types of CSCs have so far been isolated and expanded by several groups (Table 1). There is partial overlap in marker expression but the origins and exact lineage relationships among these various CSC populations are still not clear. It remains to be determined whether some or all of these cells originate from a common stem cell population and are simply at different stages of development in the heart, or whether they are independent cell lineages derived from distinct stem/progenitor cell populations. Whatever their diversity, CSCs will be an important source of cells to exploit in cardiac regeneration therapy since they are intrinsically programmed to generate cardiac tissue and to increase cardiac tissue viability.

Table 1. Overview of markers used to characterize different CSC populations
CSC descriptorSpeciesMarkersChemokine/ Cytokine receptorsReferences
  1. CSCs, cardiac stem cells; CSP, cardiac side population.

CSP cellsMouse and ratSCA1+/CD31-CXCR4Pfister et al., 2005; Liang et al., 2010; Oyama et al., 2007.
c-kit+Rat, mouse and humanc-kit+/Lin-c-kit, c-met, IFG-1R and EphA2Beltrami et al., 2003; Linke et al., 2005; Urbanek et al., 2005; Goichberg et al., 2011.
SCA1+MouseSCA1+/CD31-,Wt1+, PDGFRα+CXCR4, PDGFRαOh et al., 2003; Oh et al., 2004; Smart et al., 2011; Chong et al., 2011.
CardiospheresMouse and humanc-kit+, CD31+, CD34+c-kit, CXCR4Messina et al., 2004; Davis et al., 2009; Smith et al., 2007; Tang et al., 2009
Isl-1+ Isl-1+UnknownLaugwitz et al., 2005; Weinberger et al., 2012

SIGNALING FACTORS AND PATHWAYS INVOLVED IN MIGRATION OF CSCs FOLLOWING MI

Hypoxia and Hypoxia-Inducible Factors

Hypoxia-inducible factors (HIFs) are transcription factors expressed in response to the stress of hypoxia. Hypoxia-inducible factor 1α is the founding member of the HIF family. Deficiency of HIF-1α in mice leads to lethal cardiovascular malformations, suggesting that HIF-1α is essential for normal cardiac and vascular development during embryogenesis (Compernolle et al., 2003). Under hypoxic conditions (such as cardiac ischemia), HIF-1α becomes post-translationally stabilized and can target a series of its downstream genes that support cell survival in a hypoxic microenvironment (Semenza, 2003). These genes include, but are not limited to, vascular endothelial growth factor-A (VEGF-A), stromal cell-derived factor-1 (SDF-1), CXCR4, erythropoietin (EPO), transforming growth factor-β (TGF-β) and c-Met (Schioppa et al., 2003; Semenza, 2003; Staller et al., 2003; Ceradini et al., 2004). In addition to hypoxia, many growth factors have been identified that can elevate HIF-1α expression under normoxic conditions, including IGF-1 and HGF (Zelzer et al., 1998; Shih and Claffey, 2001; Tacchini et al., 2001). The upstream and downstream genes of HIF-1α listed above are associated with myocardial regeneration and repair after MI. Among the downstream genes, SDF-1 and its receptor, CXCR4 have been shown to play important roles in stem cell migration and homing (Ceradini et al., 2004; Staller et al., 2003). Studies by Ceradini et al. (2004) have shown that the upregulation of SDF-1 in ischemic tissues is directly proportional to the reduction of oxygen tension. The recruitment of CXCR4+ stem/progenitor cells to regenerating tissues is mediated by the hypoxic gradient through HIF-1α-induced expression of SDF-1 (Ceradini et al., 2004). This indicates that HIF-1α may be a fundamental regulator for stem cell migration following MI. Furthermore, accumulation of HIF-1α has been found in ischemic tissue after MI (Jurgensen et al., 2004). On the basis of these findings, enhancement of HIF-1α expression in the ischemic myocardium has been considered as a therapeutic strategy. Plasmid DNA encoding HIF-1α delivered by intra-myocardial injection significantly enhanced neovascularization and reduced infarct size in a rat MI model (Shyu et al., 2002). Moreover, using transgenic mice that express HIF-1α under the control of cardiomyocyte specific α-myosin heavy chain, Kido et al. (2005) have demonstrated that HIF-1α enhances angiogenesis, limits infarct size, and improves LVEF after acute coronary occlusion.

Interestingly, ischemic/hypoxic preconditioning (IPC/HPC) that subjects the animal to repeated brief episodes of induced cardiac ischemia, does not lead to cardiac cell death. Instead such preconditioning protects the heart from subsequent ischemic injury (Murry et al., 1990). Recently, Gyongyosi et al. (2010) have shown that HPC enhances migration and recruitment of hematopoietic stem cells (HSC) and mesenchymal stem cells (MSC) to infarcted myocardium in a porcine myocardial ischemia-reperfusion model.

Studies by Tang et al. (2009) have further demonstrated that HPC enhances CLK cell survival and migration to ischemic myocardium through activation of the SDF-1/CXCR4 pathway mediated by HIF-1α. These findings demonstrate the potential clinical impact of augmenting hypoxia-mediated stem cell migration and survival as therapy in myocardial infarction and ischemic heart disease. Transplantation of donor stem cells after MI has been tried in both human and animal studies as a new cellular therapy strategy. One of the major problems in this therapy is the rapid and massive loss of donor stem cells after engraftment in the infarcted myocardium (Muller-Ehmsen et al., 2002; Pagani et al., 2003) Transplantation of hypoxia-preconditioned donor stem cells after MI might provide a new strategy to overcome this problem. The protective effect of HPC has also been demonstrated in a number of other organs, including the kidney and the brain (Bernhardt et al., 2007). HIFs, therefore, are central to cellular adaptation to ischemic/hypoxic environments.

SDF-1/CXCR4

SDF-1 and its receptor CXCR4, play crucial roles in migration and homing of hematopoietic stem cells (Broxmeyer, 2008), and hematopoiesis (Broxmeyer, 2001). Similarly, SDF-1 and CXCR4 are also important in cardiogenesis and vasculogenesis. Both SDF-1 and CXCR4 knockout mice have defects in cardiac development, bone marrow hematopoiesis and organ-specific vasculogenesis, resulting in early perinatal lethality (Nagasawa et al., 1996; Zou et al., 1998). Since the expression of SDF-1 and CXCR4 is directly mediated by hypoxia/HIF-1α under the hypoxic condition (Staller et al., 2003; Ceradini et al., 2004), the SDF-1/CXCR4 pathway was thought to be important in the migration of stem cells for hypoxia/ischemia-related tissue repair, such as MI. Studies have shown that the expression of SDF-1 was up-regulated in infarcted myocardium following MI (Pillarisetti and Gupta, 2001; Vandervelde et al., 2007). We have confirmed that SDF-1 upregulated in the infarcted myocardium, but moreover, its receptor, CXCR4 was upregulated on the SCA1+/CD31- CSP cells (Liang et al., 2010). We hypothesized that the SDF-1/CXCR4 pathway might play an important role in the migration of CSP cells from noninfarcted area to the infarcted site within the myocardium where they subsequently differentiate into cells with a cardiomyocyte and endothelial like-phenotype. Tang et al. (2009) demonstrated that HPC increased CXCR4 expression on cardiosphere-derived/Lin/c-kit+ (CLK) cells via the hypoxia/HIF-1α pathway, and markedly augmented recruitment of intravenously injected CLK cells to the ischemic myocardium. Infarct size was reduced and cardiac function improved in mice administered with hypoxia-preconditioned CLK cells, compared with mice treated with cells cultured under normoxic conditions (Tang et al., 2009). These studies also showed that: (1) the increase of CXCR4 expression on the CLK cells was preceded by an increase in the expression of HIF-1α and that transfection of CLK cells with HIF-1α siRNA impaired hypoxia-induced CXCR4 expression on these cells, consistent with HIF-1 inducing CXCR4 expression; (2) CLK cell recruitment to the ischemic myocardium was largely eliminated by the addition of a CXCR4 inhibitor. This indicates that the migration of CLK cells is mediated mainly by the SDF-1/CXCR4 pathway which, in turn, is upregulated by the hypoxia/HIF-1α pathway.

In addition to the effect of the SDF-1/CXCR4 pathway on the migration of resident CSCs, many studies have shown that increase of SDF-1 expression in the ischemic myocardium enhances the recruitment of CXCR4+ bone marrow stem/progenitor cells into the infarcted myocardium, resulting in an increase of neovascularization and improvement in cardiac output and ejection fraction (Askari et al., 2003; Abbott et al., 2004). Moreover, the blockading of SDF-1 in ischemic tissue or CXCR4 on circulating cells to impair the SDF-1/CXCR4 interaction, prevents stem/progenitor cell recruitment to sites of ischemic injury such as infarcted myocardium (Abbott et al., 2004; Ceradini et al., 2004). Thus, the SDF-1/CXCR4 pathway may be one of the most important factors inducing migration of hematopoietic stem cells and nonhematopoietic CXCR4+ tissue-committed stem/progenitor cells such as resident CSCs, to the infarcted myocardium following MI.

The intracellular mechanisms by which SDF-1/CXCR4 signaling can induce cell migration have also been studied. Growing evidence suggests that multiple signaling mechanisms exist to regulate cell migration. Phosphoinositide 3-kinase (PI-3k), protein kinase C (PKC), and p38 mitogen-activated protein kinase (p38-MAPK) signaling pathways have been shown to regulate the SDF-1/CXCR4 pathway-driven migration of HSC and human umbilical cord blood MSC (Wang et al., 2000; Ryu et al., 2010). These intracellular signaling pathways might be core mediators of stem cell migration downstream of SDF-1 and CXCR4.

Stem Cell Factor/c-kit

Stem cell factor (SCF) and its receptor, c-kit, play an important role in the migration of endogenous c-kit+ CSCs to infarcted myocardium after MI. Kuang et al. (2008) have demonstrated that the expression of SCF is significantly increased in the region immediately adjacent to the infarct (the peri-infarct myocardium) following MI in rats. The authors of these studies found higher expression of SCF in peri-infarct regions associated with an accumulation of transplanted c-kit+ CSCs. In vitro experiments have suggested that SCF induces c-kit+ CSC migration in a concentration dependent manner that could be antagonized by antibody against c-kit. Furthermore, the SCF-induced migration of c-kit+ CSCs to infarcted myocardium was blocked by the inhibition of p38-MARK in vivo (Kuang et al., 2008). These results suggest that the SCF/c-kit mediate migration of c-kit+ CSCs and that migration of stem cells induced by SCF/c-kit and SDF-1/CXCR4 pathways may share downstream p38-MAPK signaling pathways. The effect of SCF on CSP cells remains uncertain since expression of c-kit is undetectable in the cells.

Hepatocyte Growth Factor/c-met

Hepatocyte Growth Factor (HGF) is a growth factor associated with proliferation of hepatocyte and other cell types (Zarnegar and Michalopoulos, 1995). Ueda et al. (2001) have shown that following MI in rat hearts, expression of HGF and its high-affinity receptor (c-met) are upregulated. HGF protected cardiomyocytes against apoptosis. Expression of c-met is directly regulated by HIF-1α under hypoxic conditions (Ceradini et al., 2004). Recent studies have demonstrated that HGF promotes proliferation and migration of resident c-kit+ CSC within the myocardium after MI (Linke et al., 2005; Urbanek et al., 2005). Those c-kit+ CSCs that expressed functional c-met were strongly attracted by a HGF gradient in vitro. Moreover, intramyocardial delivery of HGF in mice enhanced migration of c-kit+ CSCs from surrounding myocardium to the infarcted region where they formed new myocytes and improved cardiac contractile function (Urbanek et al., 2005). Interestingly, the effect of HGF on migration of c-kit+ CSCs has been attributed to the expression of matrix metalloproteinases (Hamasuna et al., 1999; Urbanek et al., 2005).

Insulin-Like Growth Factor I/Insulin-Like Growth Factor I Receptor

Insulin-like growth factor I (IFG-1) is produced in the heart and appears to be a potent cardiomyocyte growth and survival factor (Palmen et al., 2001). Mice deficient in IGF-1 display increased apoptosis of cardiomyocytes, whereas overexpression of the IGF-1 gene inhibits apoptotic cell death after MI (Li et al., 1997; Palmen et al., 2001). The c-kit+ CSCs that express IGF-1 receptor (IGF-1R) display enhanced proliferation, differentiation, and survival in response to the IGF-1/IGF-1R interaction (Linke et al., 2005; D'Amario et al., 2011). This resulted in improvement of cardiac output, ejection fraction and the left ventricular developed pressure (LVDP) after MI (Linke et al., 2005). In addition to its protective effect, studies have demonstrated that overexpression of IGF-1 in transplanted MSC leads to increased migration of stem cells at the site of MI mediated through activation of the SDF-1/CXCR4 pathway by upregulation of SDF-1 expression (Li et al., 2007; Haider et al., 2008).

Erythropoietin/Erythropoietin Receptor

Erythropoietin (EPO) produced mainly in the kidney and liver, regulates erythrocytosis. In addition to the kidney and liver, EPO mRNA has been detected in other tissues including myocardium, brain and bone marrow. The EPO receptor (EPOR) is also found in cardiomyocytes and endothelial cells (Maiese et al., 2005). EPO plays important roles in erythropoiesis, as well as in cardiogenesis (Stuckmann et al., 2003). Deficiency of either EPO or EPOR leads to a combination of anemia and cardiac failure, resulting in embryonic lethality in mice (Wu et al., 1999). In adult life, EPO is a cellular survival factor that can prevent apoptosis in ischemic diseases (Maiese et al., 2005). In vitro, EPO markedly prevented the hypoxia-induced apoptosis of cultured adult rat cardiomyocytes. Administering EPO to rats immediately after coronary occlusion (5000 IU/kg/day for 7 days) reduced cardiomyocyte loss by 50% (Calvillo et al., 2003). Recently, Klopsch et al. (2009) have demonstrated that intracardiac injection of erythropoietin recruited stem cells, and improved cardiac output and ejection fraction in a rat MI model. Studies in mice by Brunner et al. (2009) showed that EPO enhanced migration of stem cells to the ischemic myocardium and that this was mediated through upregulation of SDF-1 expression and the SDF-1/CXCR-4 pathway. Hypoxia drives increased expression of EPO via HIF-1α (Semenza, 2003). The up-regulation of EPO by HPC following MI in rats not only protected myocardium against ischemic injury, but also acted via SDF-1/CXCR-4 to increase the migration of HSC to infarcted myocardium (Lin et al., 2008). This led to significantly reduced infarct size and improved LVDP, reinforcing once again the central role of HIF-1α in hypoxia-induced stem cell migration and survival.

EphrinA1/ EphA2 Receptor

Ephrins are membrane-bound ligands of Eph receptor tyrosine kinases on the cell surface. Bi-directional interactions between ephrins and Eph-receptors mediate signaling between cells during embryogenesis and adult life (Poliakov et al., 2004). In general, ephrins are divided into A and B subclasses. A-subclass ephrins (ephrins A1–A5) bind A-subclass Eph receptors (EphA1–A8) and B-subclass ephrins (ephrinB1–B3) bind B-subclass Eph (EphB1–EphB4 and EphB6) (Pasquale, 2008). Of the five ephrinA ligands, ephrinA1 is the only ligand that binds all eight EphA receptors (Ogawa et al., 2000). Expression of both subclasses of Eph receptors and ephrins is directly regulated by HIF-1α under hypoxic conditions (Vihanto et al., 2005).

Eph receptors and their ligands appear to participate in the development of the cardiovascular system as well as angiogenesis, and vascular remodeling (Zhang and Hughes, 2006). A recent study by Dries and colleagues showed that intramyocardial administration of recombinant ephrinA1-Fc promoted myocardial tissue repair after MI (Dries et al., 2011). Following MI in mice, ephrinA1 promoted migration of EphA2-positive hCSCs (c-kit+) to the ischemic area and enhanced cardiac repair, resulting in an improvement of cardiac contractile function and reduction of infract size (Goichberg et al., 2011). Together, these studies suggest that EphrinA1/EphA2 signaling also play a role in the myocardial repair after MI.

SUMMARY

MI leads to cell death and replacement of cardiomyocytes with scar tissue. The ultimate goal of stem cell-based therapy is to restore cardiac structure and function through regeneration of functional cardiomyocytes. This may be achieved by the resident CSCs, the therapeutic potential of which depends upon their migration from surrounding tissue to the infarct area where they undergo proliferation, differentiation and subsequently repair the damaged myocardium. Several pathways are involved in regulating the migration of different populations of CSCs. The SDF-1/CXCR4 pathway seems to play a fundamental role in migration of cardiac SP cells, SCA1+ cells, and CDCs. On the other hand, migration of c-kit+ CSCs is mediated by HGF/c-met, SCF/c-kit and EphrinA1/EphA2 pathways. Interestingly, migration of CSCs induced by SCF/c-kit and SDF-1/CXCR4 pathways may share similar intracellular mechanisms, both working via the p38 MAPK signaling pathway.

Hypoxia-inducible transcription factors directly mediated by hypoxia and HPC (in particular HIF-1α), may play important roles in regulation of CSCs migration, proliferation, and cytoprotection since they are the direct upstream regulator of many genes involved in these functions (Fig. 1). Therefore, targeting HIF-1α might enhance the migration and proliferation of CSCs, and thereby promote cardiac regeneration and repair in ischemic heart disease. Hypoxic preconditioning treatments may provide a novel strategy to drive the HIF-1α-mediated pathways and enhance the therapeutic efficiency of stem cell therapy for cardiac regeneration and repair by increasing resident CSCs migration and proliferation. Transplantation of hypoxia-preconditioned donor stem cells after MI, if translated into clinical therapy, might hold enormous potential for therapeutic myocardial regeneration in ischemic heart disease.

Figure 1.

Schematic overview of the key factors and pathways involved in migration of CSCs within myocardium following MI. We hypothesize that HIF-1 (the central black box in the figure) may serve as a key mediator for migration of CSCs following MI because the majority of genes involved in migration of CSCs are transcriptionally regulated by HIF-1. The large arrows represent what may be the central hypoxic response pathway involved in CSC migration. The SDF-1/CXCR4 pathway may play a fundamental role in CSCs migration because CSP cells, SCA1+ cells and CDCs are all induced to migrate by this pathway.

Migration of resident CSCs is controlled by complex signaling pathways. Understanding these signaling processes will facilitate the development of new strategies to enhance the migration of resident CSCs to ischemic myocardium. This will take us one step closer to the ultimate goal of therapeutic myocardial regeneration in ischemic heart disease.

Acknowledgements

The authors thank Mr. Tim Dai for his critical review of the manuscript.

Ancillary