BADANIA KLINICZNE I DOŚWIADCZALNE W CHOROBACH SERCA, PŁUC I NACZYŃ
Scaffold construction for effective transfer of cardiac stem cells to the damaged heart

Introduction

In recent years, much work has focused on elucidating the biology of stem cells and their mechanisms of action with a view towards using them to regenerate damaged organs, including the treatment of myocardial infarction. For this reason, great interest and expectations are associated with the discovery of cardiac stem cells (CSCs), a group

of newly discovered multipotent cells that are capable of multiple rounds of replication and differentiation into cardiomyocytes, endothelial cells and smooth muscle cells. These cells are characterized by the presence of the surface proteins c-kit (CD117), MDR-1 and Sca-1. The discovery of CSCs revolutionized the perception of the heart muscle as an organ incapable of regeneration. Currently, it is believed that aging heart muscle cells, including cardiomyocytes, coronary vessel-forming cells and stromal cells, are constantly replaced by new generations of corresponding cells that are fully functional. However, the great potential for cardiac regeneration appears to be insufficient with respect to ischemia. It is possible that the microenvironment of the infarcted heart inhibits the recruitment of residual stem cells. Because the treatment of pathologically damaged heart muscle by cell therapy involves repopulating the infarcted region with a sufficient number of CSCs, these cells can be isolated from heart muscle and expanded in vitro. However, the effectiveness of this type of therapy is largely determined by the efficiency with which these cells localize to the damaged heart muscle. The use of the scaffolds is one method of administering stem cells to the damaged heart. Note that the microenvironment of the cells, direct signal transduction by matrix molecules through integrin receptors and physical properties, such as scaffold stiffness, can influence the fate of stem cells and their differentiation process. The aim of this study was to evaluate the impact of different types of substrata used in the preparation of the scaffold on the growth ability and surface receptor expression of CSCs. Our results should contribute to the development of an appropriate scaffold dedicated to effective CSC transfer to damaged heart muscle.

Material and methods

Cardiosphere-forming cell isolation and culture



Cardiac stem cells were isolated using the enzymatic method according to Messina and colleagues. Briefly, cardiac muscle tissue derived from explanted heart was cut into 1-2 mm3 pieces and washed in phosphate buffered saline (PBS) without Ca2+ or Mg2+ several times to remove the contaminating fat and red blood cells. After washing, the tissue was digested using a solution of 0.2% trypsin/EDTA plus 0.1% collagenase IV for 5 min at 37°C; the procedure was repeated 3 times. After isolation, the cells were discarded, and the remaining tissue fragments were washed in IMDM medium (Iscove’s Modified Dulbecco’s Medium) supplemented with 10% fetal calf serum (FCS), 20 U/ml penicillin G, 20 mg/ml streptomycin and 2 mmol/L L-glutamine. After washing, the tissue fragments were seeded onto culture dishes and incubated at 37°C with 5% CO2. After a period of approximately 2-3 weeks, a fibroblast-like layer was generated, over which small, round, phase-bright cells were observed. The phase-bright cells were collected through two washes in PBS without Ca2+ or Mg2+ (1-2 min), one wash in 0.53 mmol/l EDTA, and finally one wash in a 0.5 g/l trypsin and 0.53 mmol/l EDTA solution (2-3 min) at room temperature. The harvested cells were transferred to culture plates coated with poly-D-lysine and cultured in a mix of IMDM medium (35%) and Dulbecco’s Modified Eagle Medium (DMEM)/Ham’s F12 medium (65%) supplemented with 1% L-glutamine, 20 ng/ml bFGF, 2% B27,

4 ng/ml cardiotrophin, 10 ng/ml EGF, 40 nM/l thrombin, and 0.2% mercaptoethanol.



Secondary culture of cardiosphere-forming cells on different substrata



After one week, cells that failed to adhere to the poly-D-

lysine coated dishes were collected by centrifugation and treated with StemPro Accutase Cell Dissociation Reagent in accordance with the manufacturer’s protocol to dissociate the cardiospheres into single cells. Afterwards, the cells were plated onto dishes coated with fibronectin, collagen IV, gelatin and laminin and cultured with a) IMDM medium (35%) and DMEM/Ham’s F12 medium (65%) supplemented with 1% L-glutamine without growth factors or b) IMDM medium (35%) and DMEM-Ham’s F12 medium (65%) supplemented with 1% L-glutamine, 20 ng/ml bFGF, 2% B27,

4 ng/ml cardiotrophin, 10 ng/ml EGF, 40 nM/l thrombin, and 0.2% mercaptoethanol.



Flow cytometry



Cardiosphere-forming cells from secondary cultures on poly-D-lysine and cells cultured on fibronectin-, collagen IV-, gelatin- and laminin-coated dishes and cultured with a) IMDM medium (35%) and DMEM/Ham’s F12 medium (65%) supplemented with 1% L-glutamine without growth factors or b) IMDM medium (35%) and DMEM-Ham’s F12 medium (65%) supplemented with 1% L-glutamine, 20 ng/ml bFGF, 2% B27, 4 ng/ml cardiotrophin, 10 ng/ml EGF, 40 nM/l thrombin, and 0.2% mercaptoethanol were prepared for flow cytometry using a Beckman Coulter FC 500 flow cytometer with MXP software (Beckman Coulter). The following monoclonal antibodies were used: c-kit, Nkx 2.5, MDR-1, GATA-4, CD90, CD45, CD31, CD73, CD105, CD144, Lin-1, Lin-2, and Lin-3.



Analysis of cell morphology – fractal analysis



Phase contrast microscopy was used to directly follow cell morphology in cultures over a period of 8 days. Images were taken 24 h after seeding the cells on fibronectin-, collagen IV-, gelatin- and laminin-coated dishes and on the 8th day of the experiment. An inverted AxioObserver microscope was used for image acquisition, and AxioVision 1.4 software was used for image processing. Additionally, binary (black and white) image processing was performed using a public domain NIH image program (ImageJ) developed at the US National Institute of Health. To quantify the complex morphology of the cells in culture, fractal dimension (FD) analysis via the box-counting method was performed using Fractalyse 2.4.1 software.



Statistical analyses



The results were reported as mean values ± SD. The basic statistics employed the STATISTICA 7.0 package. The data were tested for homogeneity of variance using Levene’s test of equality of error variances. Normality was checked by the Kolmogorov-Smirnov test. Whenever a significant effect was observed, post hoc comparison was performed with the Tukey test to check differences between experimental groups.

Results

Primary and secondary cell culture



Explanted human hearts were stored in high-potassium cardioplegic solution and processed within 1 h. The tissue was cut into 1-2 mm3 pieces, and the isolation procedure was performed. After a period of approximately 2-3 weeks in the primary culture, a layer of fibroblast-like cells was observed over which small, round, phase-bright cells were observed. The phase-bright cells were collected by mild digestion and transferred to poly-D-lysine-coated dishes. After seeding on poly-D-lysine-coated dishes, the cells formed a cardiosphere (Fig. 1).

The cardiospheres in secondary culture on poly-D-lysine-coated dishes were loosely adherent. The cardiosphere-

forming cells were c-kit-positive in the core and CD105-positive in the periphery.

Flow cytometric analysis of the cells in the secondary culture indicated that nearly 7% of the cells were c-kit positive and that approximately 6% were Nkx-2.5 positive. Expression of the MDR-1 and GATA-4 receptors was observed in the secondary culture (Fig. 2). Approximately 80% of the cardiosphere-forming cells were CD90-positive, and these cells were also largely (approximately 90%) CD105- and CD144-positive. The CD90-, CD105- and CD144-positive cells presented a mesenchymal or fibroblast phenotype (Fig. 3). Only a low percentage of the cardiosphere-forming cells were positive for the hematopoietic lineage markers CD45, CD31, Lin-1, Lin-2, and Lin-3 (Fig. 4).



Cardiosphere-forming cells after transfer to different substrata: fibronectin, collagen IV, laminin, gelatin



Cells derived from the secondary culture on poly-D-lysine-coated dishes were dissociated using Accutase to a single cell suspension and transferred to culture dishes coated with fibronectin, collagen IV, laminin or gelatin. Additionally, after being transferred to the different substrata, the cardiosphere-forming cells were divided into two groups: one group cultured without growth factors and a second group in which the culture medium was supplemented with growth factors (Figs. 5-8). Differences in cell morphology between the treatment groups were monitored. After 24 h, the morphology of the cells in the two groups was similar. Phase contrast microscopy revealed spindle-shaped cells, among which single, small, phase-bright, round cells were observed. On the laminin-coated dishes after 24 h, cardiosphere structures were observed in both the growth factor-supplemented and non-supplemented groups. After a period of 8 days, the morphology of the cells in these two groups differed significantly (Fig. 7). With the exception of the fibronectin group, in which the formation of cardiospheres and capillary-like structures was observed in the group cultured with growth factors, spindle-shaped cells and single, small round cells were observed after 8 days of culture with growth factors. In contrast, the cells cultured on the same substratum but without growth factors formed a monolayer of small, round, phase-bright cells (Figs. 5-8). A monolayer of small round cells was also observed in the gelatin-coated dishes cultured with growth factors (Fig. 8).

In the culture of cardiosphere-forming cells cultured with growth factors, the number of cells after 8 days of culturing was nearly the same as after 24 h; no significant subsequent proliferation was observed. In contrast, in the same time period, a significant increase in the number of cells

was observed in the group cultured with the same substratum but without growth factors. This result indicates a significantly higher proliferation rate in this group than in

the group cultured with growth factors.



Fractal dimension analysis



To quantify the complex morphology of the cells in culture, fractal dimension (FD) analysis was performed via the box-counting method. After 24 h of culture, the FD was nearly the same in the cells cultured with growth factors and in those cultured on the same substratum but without growth factors. The initial FD was approximately 1.5 in all of the investigated groups. After 8 days of culturing, the FD increased significantly in the cells cultured without growth factors, reaching a value of approximately 1.8. Thus, the FD increased by approximately 20% (Figs. 9-12). In the sole case of cells cultured on gelatin-coated dishes without growth factors, the FD increased by 10%, as the initial value after 24 h was higher than in the other study groups and the FD was 1.6 (Fig. 12). In the case of cells cultured with growth factors, the FD increased only slightly by approximately 2-5% during the 8 days of culturing (Figs. 9-12). Significant differences between the cells cultured with growth factors and without growth factors were visible starting from day 6 of the culture. This finding may indicate greater complexity of the cultures not supplemented with growth factors.



Percentage of c-kit- and MDR-1-positive cells in cultures on different substrata with and without growth factors



After a period of 8 days, the percentage of c-kit- and MDR-1-positive cells differed significantly depending on the culture conditions. The initial proportions of c-kit- and MDR-1-positive cells in the secondary culture on poly-D-lysine were 6.9% and 4.4%, respectively. In the case of cells cultured without growth factors on collagen IV-coated dishes, the percentage of c-kit-positive cells was 7.1%. This value was not significantly different from that obtained on poly-D-lysine, but the percentage of MDR-1-positive cells was higher on poly-D-lysine, reaching a value of 9%. In the case of cells cultured on laminin- and gelatin-coated dishes, the percentages of both c-kit- and MDR-1-positive cells were lower in relation to the values on poly-D-lysine. Significantly greater percentages of c-kit- and MDR-1-positive cells were observed on the fibronectin-coated dishes, reaching values of approximately 14% for the c-kit receptor and 21% for the MDR-1 receptor (Fig. 13).

In the case of cells cultured with growth factors on collagen IV-, gelatin- and fibronectin-coated dishes, the percentage of cells expressing the c-kit receptor was insignificantly higher than that obtained on poly-D-lysine. In contrast,

the c-kit-positive cells in the group of laminin-coated dishes decreased approximately 2-fold in relation to growth on poly-D-lysine. The percentage of MDR-1 positive cells was approximately 3.2% on collagen IV-coated dishes and approximately 1.8% on laminin-coated dishes, which are values lower than those obtained on poly-D-lysine. Similar values for the MDR-1 receptor were observed for the cells cultured on gelatin- and fibronectin-coated dishes (Fig. 13).

Comparing the cells cultured with growth factors to those cultured without growth factors in collagen-coated dishes, the percentage of c-kit positive cells was roughly the same, but for the MDR-1 receptor, the value obtained for the growth factor group was approximately two-fold lower. In the groups of cells cultured on laminin- and fibronectin-coated dishes without growth factors, higher percentages of c-kit- and MDR-1-positive cells were observed. In contrast, for the gelatin group, higher values for both receptors were observed in the cell cultures supplemented with growth factors.

Discussion

Cells capable of self-renewal and bearing the potential of developmental plasticity have recently been shown to reside in the myocardium. These (CSCs/CPCs) are multipotent and clonogenic, giving rise to cardiomyocytes, smooth muscle cells and endothelial cells both in vivo and in vitro. The use of CSCs/CPCs in cell therapeutic applications for the reconstruction of ischemically altered heart tissue is very promising. However, the effectiveness of cell therapies strongly depends on methods for administering stem cells to the target area. The type of stem cell transfer depends on many factors, such as the clinical scenario. In the case of acute heart disease, the cells can be administered peripherally or locally through the circulatory system. In chronic ischemia and non-ischemic cardiomyopathy, the preferred method is direct injection into the heart muscle [1]. One

of these methods is surgical intramyocardial injection, which can be performed during routine surgical procedures. The advantages of this type of administration include the ability to directly visualize and inspect the injection area. In a study of Herreros et al. [2], patients with a previous myocardial infarction and ischemic coronary artery disease underwent the injection of skeletal myoblasts during a coronary artery bypass procedure. The authors concluded that the intramyocardial injection of cells combined with a coronary artery bypass is safe; they observed increases in both global and regional left ventricular function and no induction of arrhythmias. The other method of stem cell administration is transendocardial injection. This method utilizes the NOGA system for the precise mapping of the border zone of the infarct [3]. Fuchs et al. observed that the transendocardial injection of bone morrow cells induces endothelial cell proliferation, stimulates angiogenesis and augments the perfusion of ischemic myocardium. Alternatively, stem cell administration can be monitored by MRI, echocardiography or computed tomography. Intracoronary injection of stem cells has been successfully performed in clinical trials. In the study of Strauer et al., autologous mononuclear bone marrow cells were transplanted using a balloon catheter placed in an infarct-related artery. They observed that the intracoronary injection of autologous cells can be safe and clinically effective [4]. The simplest method of stem cell transfer is intravenous injection. Boomsma [5] demonstrated that intravenously injected murine mesenchymal stem cells are able to target the myocardium, attenuate cardiac dysfunction and myocardial remodeling and preserve systolic function. All

of these methods of stem cell administration to the infarcted area have some limitations. Intracoronary administration is not suitable for larger cells such as myoblasts because of the risk of embolization of the cells, impaired coronary flow and/or myocardial cell necrosis. With the intravenous method, there is the problem of targeting

the cells and that only a small portion of the cells enter

the coronary circulation and migrate into the ischemic myocardium [1]. It is also important to note that there are limited data in the literature concerning methods of administering CSCs/CPCs for the treatment of heart disease.

Because of these disadvantages, the optimal route of ad-

ministering stem cells, such as CSC/CPC administration, is still being sought. One technique that can improve cell therapies and tissue engineering applications is the combination of stem cells with a scaffold composed of natural or synthetic materials. The use of a scaffold for stem cell therapy, including heart disease treatment, is attractive because it harbors the stem cells in an environment that mimics normal cellular conditions. In our study on the culture of car-

diac stem/progenitor cells, we used natural components

of the extracellular matrix (ECM) as starting points for developing scaffolds based on natural materials. We cultured the CSCs/CPCs on fibronectin-, collagen IV-, laminin- and gelatin-coated surfaces. We decided to test these factors and their influence on the morphology and phenotype

of CSCs/CPCs because the extracellular matrix is composed of three major proteins: fibronectin, collagen and laminin. It is well known that stem cells require the local microenvironment of their niche to develop [6, 7]. The ECM is the main component of the niche, and it is involved not only in mechanically supporting stem cells but also in the regulation of their proliferation, migration and differentiation [8].

In the absence of growth factors, we observed no significant difference in the numbers of c-kit- or MDR-1-positive cells on collagen IV-coated dishes when compared with the results obtained on poly-D-lysine. In the case

of cells cultured on laminin or gelatin, a slight decrease in the percentage of c-kit- and MDR-1-positive cells was observed relative to the results obtained on poly-D-lysine. The op-

posite results were obtained with the fibronectin-coated dishes, on which we observed a roughly three-fold increase in both c-kit- and MDR-1-positive cells. These differences may be caused by the fact that the extracellular matrix components used in the CSC/CPC cultures differ in their structural, biochemical and mechanical properties and that the stem cells react to all these stimuli. In a study by Rowlands et al. [9], when mesenchymal stem cells (MSCs) were cultured on a collagen IV- or laminin-coated surface, only poor induction of differentiation was observed. However, when the MSCs were cultured on a stiffer scaffold with elasticity > 30 kPa and on collagen I or fibronectin coupled to the surface,

the production of early osteogenesis markers was observed, suggesting a role in differentiation of the complete environment. The structure of the components can also affect how stem cells differentiate. With a collagen-based structure, MSC osteogenesis is inhibited when the cells are cultured on collagen gel, but this inhibition is not observed when the cells are cultured as a monolayer on collagen-coated dishes [10]. It is interesting that differentiation depends not only on the type of the substratum but also on its mechanical properties, such as elasticity. It was observed that embryoid body formation is inhibited when the collagen gel stiffness is approximately 34 kPa, but when the scaffold is more flexible (16 kPa), inhibition is not observed [11]. In the same study by Battista et al., the induction of endothelial cells was observed when soft collagen was supplemented with fibronectin, but after laminin addition, the embryonic stem cells differentiated into cardiac muscle cells.

The myocardium is composed of cardiac cells placed in the extracellular matrix. As with other types of stem cells, the use of molecules such as collagen, laminin, and fibronectin in in vitro cultures can induce the differentiation

of adult cardiac stem cells into cardiomyocytes. Moreover, the activation of spontaneous contractility and morphological differentiation into neonatal cardiomyocytes can be observed in cell culture [12]. It has been observed in both in vitro and in vivo studies that not only separate ECM elements but also a mixture of these components can stimulate the dif-

ferentiation of stem cells into cardiomyocytes. In a paper by Bick et al., a beneficial effect of Cardiogel, an ECM mixture composed mainly of laminin and fibronectin, on the differentiated morphology of primary cardiomyocytes was reported [13]. In a study by Leor et al. [14], fetal cardiac myocytes were seeded on an alginate scaffold and cultured for 4 days. This cardiomyocyte-seeded scaffold was implanted into a rat model 7 days after myocardial infarction. They observed that the 3D alginate scaffold provided a good environment for cardiomyocyte growth. After implantation into the infarcted heart, neovascularization of the left ventricle (LV) and attenuation of LV dilatation were observed. In a study by Akhari, heart cells were seeded onto a gelatin matrix and subjected to cyclical mechanical stress using the Bio-Stretch Apparatus. The use of cyclical stretching improved the proliferation and distribution of the seeded cells, and the formation and organization of the matrix were enhanced [15]. Biomaterials used in regenerative medicine applications can serve not only as a scaffold for cell proliferation and differentiation but also to capture stem cells, thereby providing enrichment for autologous cells at the target area. One of these methods conjugates Sca-1 antibody, which specifically recognizes cardiac cells, to a collagen scaffold. This scaffold with a covalently conjugated antibody can effectively regenerate heart tissue when transplanted into the infarcted area [16].

To explain the differences in c-kit- and MDR-1-positive cells observed with different substrata, it is important to note that there are multiple mechanisms involved in

the ECM-based control of stem cell fate, including the geometry and elasticity of the ECM and mechanical signal transmission from the ECM to cells [17]. Knowledge regarding these mechanisms is very important for the proper construction of a scaffold on which cardiac stem cells can be seeded, and it is also important for understanding the behavior of cardiac stem cells after implantation into the diseased

heart. The influence of ECM stiffness on cell fate was revealed in the study by Emerman et al. [18], in which it was observed that mouse epithelial cells display increased differentiation when cultured on soft collagen compared with the normal plastic surface of culture dishes. In another study, myoblasts cultured on polymer gel developed actin/myosin striations, and this effect was not observed with myoblasts cultured on a collagen surface [19]. In another study by Engler et al., MSCs were cultured on substrates with different stiffnesses; the cells cultured on the soft scaffold differentiated into neurons, those cultured on the intermediate scaffold differentiated into myoblasts, and the stiff substratum promoted the differentiation of MSCs into osteoblasts [20]. The nanotopography of the ECM may be another factor that influences stem cell fate. It seems that these cells have the ability to sense micro- and nanotopographical changes in the ECM and to react to these changes. The effects of tissue-specific matrix orientation on stem cell differentiation have been investigated by Yin et al. They seeded human tendon stem/progenitor cells (hTSPCs) on aligned and randomly oriented poly-L-lactic acid nanofibers. The cells seeded on the aligned fibers were spindle shaped, well oriented and displayed higher expression

of tendon-specific genes compared to the cells seeded on the randomly oriented fibers. In the same study, they observed that the randomly oriented fibers induced osteogenesis but that the aligned fibers hindered this process [21]. Throughout their life, cells are exposed to mechanical stimuli, and it is believed that cells are not passive biomaterials but instead react to mechanical factors, including ECM mechanical stimuli. In response to these forces, cells remodel their cytoskeleton, which is one of the effects of the stem

cell differentiation response [17]. In a study by Arnsdof et al. [22], oscillatory fluid flow as a mechanical signal was examined, and the role of this signal on the osteogenic, chondrogenic and adipogenic differentiation of murine mesenchymal stem cells was determined. They hypothesize that isomeric tension within the actin cytoskeleton induced by fluid flow is one of the essential factors involved in differentiation. The authors found that mechanical stimuli upregulated the genes Runx2, Sox9 and PPARg, which are critical for cell differentiation. In a study by Gong [23], MSCs were cultured on different protein-coated flexible membranes. When they additionally exposed the cells to 5% or 10% cyclic uniaxial stretching, the myogenic phenotype and the expression of smooth muscle actin (SMA) were observed.

An important factor involved in the regulation of cell fate (including stem cell fate) is cell shape. Changes in shape may be involved in myocardial development [24]. It was observed by Holzer et al. [25] that chondrocytes cultured in flattened 2D conditions dedifferentiate and adopt a fibroblastic morphology. The same cells cultured in 3D conditions in agarose or alginate gel retain normal chondrocyte morphology [26]. Because of this, in our study, the influences of different substrata, including fibronectin, collagen IV, laminin and gelatin, on the changes in cell shape were analyzed. Additionally, the effect of growth factor supplementation on cell morphology was analyzed. The tools available for analyzing cell morphology and cell shape in populations of in vitro cultured cells are not ideal. As an attractive alternative, fractal analysis can be used to quantify changes in the morphology of in vitro cultured cells. Limited data exist regarding the morphology and shape of cardiac stem cells cultured on different substrata in different conditions. In our study, we monitored the morphology of cells cultured on different substrata both with and without growth factor supplementation. The general tendency observed was that the FD of the cells increased significantly (by approximately 20%) in the secondary cultures of CSCs/CPCs cultured without growth factors, and in the cells cultured with growth factors, there was only a slight increase (approximately 4%) in the FD value. In a study by Pearson [27], image and fractal analyses were used to determine how the viscosity

of the culture medium affects osteoblast morphology. In that study, cells were grown in culture medium supplemented with methyl cellulose (MeC). The authors demonstrated that the medium viscosity can affect cell morphology and that fractal analysis can be used to qualitatively analyze these changes and to classify the cells based on morphology. In a paper by Bernard [28], Mandelbrot’s fractal dimension was used to quantify the morphology of glial cells. They observed that when the morphological complexity

of the cells increases, their FD value increases, and it was hypothesized that the FD value can be used to determine the stage of differentiation. In another experiment, they calculated the FD value for rat cerebellar oligodendrocytes during differentiation in primary culture. These data were correlated with the expression of A2B5, O4 and anti-galactocerebroside (Gal-C), which are commonly used differentiation markers. Their results indicate that the FD value can be used to determine the stage of differentiation.

In our study, we also observed trends involving the FD value and the percentages of c-kit- and MDR-1-positive cells. When the FD value increased during a day of culturing, increases in c-kit- and MDR-1-positive cells were also observed. An increase in the FD indicates an increase in the complexity of the cell culture. Additionally, in conjunction with phenotypic calibration, FD values can allow the stage of cardiac stem cell differentiation to be determined, which can be useful in the construction of a dedicated scaffold for use as a cell delivery system in the treatment of diseased hearts.

Because of their potential clinical applications, the ideal culture medium for the growth, proliferation and differentiation of cardiac stem cells should be serum-free and supplemented with specific growth factors. However, cells cultured in serum-free media display a lower mitotic index, whereas apoptosis and pure adhesion are stimulated [29]. Because of this, it is important to supplement the culture medium with growth factors that promote the proliferation and growth of the cells. Depending on the final application, the growth factors chosen can promote the differentiation or expansion of the stem cells.

In our study, we cultured CSCs/CPCs on different substrata in culture medium without growth factors and on the same substratum but in culture medium supplemented with 20 ng/ml bFGF, 2% B27, 4 ng/ml cardiotrophin, 10 ng/ml

EGF, 40 nM/l thrombin, and 0.2% mercaptoethanol. We observed that with the collagen IV-, laminin- and fibronectin-coated dishes, the groups supplemented with growth factors presented fewer c-kit- and MDR-1-positive cells than the groups cultured without growth factors. Specifically, significant differences were observed between the fibronectin groups, in which approximately two-fold and four-fold fewer c-kit- and MDR-1-positive cells, respectively, were observed. Only for the laminin substrate was a slight increase in the percentage of c-kit- and MDR-1-positive cells observed in cultures supplemented with growth factors. This result indicates that although the use of growth factors can induce the differentiation of CSCs/CPCs, to expand the number

of undifferentiated c-kit-positive cardiac stem cells, culturing without growth factors is the preferred method.

The study by Urbanek et al. revealed that most CSCs express c-Met, insulin-like growth factor-1 (IGF-1) and hepatocyte growth factor (HGF). After injecting IGF-1 and HGF into infarcted hearts, mobilization of CSCs to the damaged area was observed, and the mobilized CSCs grew, survived and proliferated [30]. HGF promotes cell migration [31]. IGF-1

acts as an antiapoptotic and mitotic factor in the infarcted myocardium, where it promotes myocyte formation and the proliferation and survival of cardiac cells [32]. Additional growth factors involved in normal heart development are a part of the fibroblast growth factor (FGF) family. In a study by Chan et al., embryonic stem cells (ES cells) and induced pluripotent stem cells (iPS cells) were cultured with the growth factors FGF-2, FGF-3, FGF-6, FGF-7, FGF-10 and FGF-15. They observed an impact of FGF-10 on differentiation into cardiomyocytes for both ES and iPS cells [33].

In another study, when ES cells were cultured with bone morphogenetic protein 4 (BMP4), basic fibroblast growth factor (bFGF), vascular endothelial growth factor (VEGF) and dickkopf homolog 1 (DKK1) in serum-free medium, a KDRlow/

C-KIT(CD117)neg population was generated that can differentiate into cardiac, endothelial and vascular smooth muscle cells in vitro [34]. Another factor used in the culture of CSCs/CPCs is epidermal growth factor (EGF). EGF interacts with the adrenergic receptor to decrease the response to epinephrine, and EGF can protect the heart from the harmful effects of epinephrine [35]. In an in vitro study, it was observed that in cultures of ventricular cardiomyocytes and noncardiomyocytes from neonatal rat hearts, EGF increased cAMP accumulation in a time-dependent manner in the cardiomyocytes but not in the noncardiomyocytes [36]. In the study by Messina, cardiac stem cells were cultured on poly-D-

lysine-coated dishes and supplemented with thrombin. During the first week of growth, this factor led to a 7-fold increase in the number of cardiospheres [37], but the mechanism by which the number of spheres increased in the culture remained unclear. One mechanism that can explain the role of thrombin in cell cultures is that thrombin can stimulate nuclei to re-enter S-phase via the phosphorylation of retinoblastoma protein [38]. An important factor that can be used when culturing adult cardiac stem cells is cardiotrophin-1 (CT-1). Cardiotrophin-1 is a member of the IL-6 family of cytokines, which are expressed in the adult heart, where they induce hypertrophy and enhance the survival of cardiomyocytes. As described by Chen, CT-1 has an effect on the differentiation of bone marrow mesenchymal stem cells (BMMSCs). Cultures of BMMSCs induced with 5-azacytidine (5-aza) and supplemented with CT-1 were found to differentiate into cardiomyocytes. In that study, the expression of GATA‑4, Nkx‑2.5, alpha-actin and beta-MHC mRNAs was observed, confirming the role of CT-1 in the differentiation and maturation of cardiomyocyte-like cells. In another study [39], a protective effect of CT-1 against hypoxia/ischemia was observed both in vitro and in vivo. Cultured adult rat cardiocytes were examined, cardiac cells were exposed to hypoxia followed by reoxygenation, and CT-1 was administered prior to hypoxia. CT-1 was found to have a protective effect, reducing ischemic damage in the cultured cells.

Conclusion

The proper growth, migration and differentiation of cardiac stem cells depends on many factors, such as the interaction of the cells with extracellular matrix components, which act not only as a support for the cultured cells but also actively influence their proliferation, migration, adhesion and differentiation. These parameters should be considered in the construction of a scaffold material to be used for the effective delivery of cardiac stem cells to the damaged heart. Additionally, the use of an appropriate combination of growth factors is another important element in directing cardiac stem cells to differentiate into cardiomyocytes, endothelial cells or vascular smooth muscle cells. From the clinical point of view, it is important to select an appropriate cell therapy strategy. Most papers have focused on the use of undifferentiated cells, but it is possible that some degree of differentiation is beneficial for certain stem cell therapy applications.

Acknowledgements

The project is financed by scientific-research grants KNW-1-181/10 and KNW-1-180/10

from the Silesian Medical University, the European Union through the European Regional Development Fund, and the State budget under the Innovative Economy Operational Programme for 2007-2013.

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