Abstract:
This invention relates, e.g., to a method for differentiating mammalian (e.g., human) pluripotent stem cells (PSCs) into endothelial cells (ECs) in vitro, by plating a single-cell suspension of PSCs onto a suitable surface such as type IV collagen and culturing the cells with VEGF after which ECs can be harvested. A preferred embodiment of the method first cultures the cells without VEGF and then sequentially cultures the cells with VEGF. Differentiation can be enhanced by adding an inhibitor of transforming growth factor β to the culturing with VEGF.

Description:
CROSS-REFERENCE TO PRIOR APPLICATIONS 
       [0001]    The present application is based on and claims the benefit of U.S. Provisional Patent Application No. 61/604,884 filed on 29 Feb. 2012 which application is incorporated herein by reference to the extent allowed by applicable law and regulation. 
     
    
     U.S. GOVERNMENT SUPPORT 
       [0002]    The present invention was partially supported by the following grants: NIH grant F31HL112644, NIH grant 2R01 HL073781, NIH grant R01HL107938 and NIH grant U54CA143868 and National Science Foundation grant 1054415; the United States government may have right to this invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    1. Area of the Art 
         [0004]    The present invention is in the area of pluripotent stem cells and more particularly deals with a method to differentiate endothelial cells from stem cells. 
         [0005]    2. Description of the Background Art 
         [0006]    Recreating functional vasculature is a pivotal step in the development of novel therapies in the field of regenerative medicine by providing innovative treatment options for patients suffering from vascular disorders through generating functional and transplantable tissues that have been engineered in vitro. The vascularization of tissue constructs remains a major challenge in regenerative medicine. Without its own blood supply, an engineered construct relies mainly on diffusional oxygen supply, which can only support a thin layer of viable tissue. Therefore, vascularization of a tissue construct is crucial for its successful implantation, survival, and integration with the host tissue. The formation of mature and functional vascular networks requires interaction between endothelial cells (ECs) and vascular smooth muscle cells (v-SMCs). During early vascular development, ECs line the vessel wall and organize into an immature vasculature. To further stabilize these nascent vessels, ECs secrete platelet-derived-growth-factors (PDGF) to induce the differentiation of specialized mesenchymal stem cells (MSCs) into pericytes in capillaries or SMCs in larger vessels. At this later stage, transforming growth factor-beta 1 (TGF-β1) regulates vessel maturation by inducing v-SMC differentiation and the generation of extracellular matrix (ECM) molecules, such as collagen, fibronectin, and Laminin. This process of vascular morphogenesis involving ECs interacting with both the ECM and v-SMCs has been widely studied in vitro using Matrigel assays. When grown on Matrigel, a basement membrane matrix enriched with laminin, ECs and v-SMCs interact to form capillary-like structures (CLSs) that resemble tube formation in vivo. 
         [0007]    ECs comprise the inner lining of blood vessels and must work dynamically with blood flowing within and with SMCs, which surround and provide support to the endothelial lining of the vessel. Obtaining a well-defined and homogenous population of ECs remains a major roadblock toward the goal of vascular reconstruction. The self-renewal capability and pluripotency of human pluripotent stem cells (hPSCs)—i.e., human embryonic stem cells (hESCs) and human induced PSCs (hiPSCs)—in vitro make them attractive for tissue engineering and vascular regenerative applications. Thus, controlled and robust differentiation of hPSCs toward vascular lineages is critical for the advancement and future of patient-specific vascular therapeutics. 
         [0008]    Both hESCs and hiPSCs have the ability to differentiate into ECs (1, 2). Many methods to induce vascular differentiation have relied on an embryoid body (EB) intermediate, which entails spontaneous differentiation to an amalgamated cell mass in suspension and subsequent isolation of cells from the EB based on specific early endothelial markers (3, 4). Other methods incorporate of inducing vascular differentiation include co-culture with mouse stromal cells, (5, 6) which is not conducive to clinical translation. By avoiding an EB intermediate and dependence on a mouse feeder layer, our previously established method guides hPSCs differentiation toward vascular lineages in an adherent culture (7). 
         [0009]    The current invention solves these shortcomings by deriving a homogenous population of ECs from hPSCs in a controllable and clinically relevant manner. We exploit the advantages of the adherent culture method to generate a highly homogenous EC population from hPSCs by using defined chemical compositions. 
     
    
     
       DESCRIPTION OF THE FIGURES 
         [0010]      FIG. 1  is a schematic diagram comparing differentiation methods; 
           [0011]      FIG. 2  shows a number of graphs showing the determination of marker expression profile of derived cells;  FIG. 2A  shows the analysis of VEcad, CD31, KDR, Tra-1-60 protein expression after 12 days in culture in media supplemented with 50 ng/ml VEGF, 50 ng/ml VEGF+SB431542, or 1 ng/ml VEGF+SB431542;  FIG. 2B  shows the expression of SMC markers over 12 days of culture;  FIG. 2C  shows representative flow cytometry plots of co-expression of VEcad with EC markers, CD31, CD105, and CD146, and pericyte marker, CD73; 
           [0012]      FIG. 3  shows immunofluorescence analysis of derived ECs examined for (A) VEcad (red), vWF (green); (B) lectin (red), CD31 (green); (C) AcLDL (red) uptake and (D) tube formation on Matrigel; nuclei in blue in (A), (B) and (C); 
           [0013]      FIG. 4  shows I functionality of hiPSC-derived ECs seven days after subcutaneous implantation of derived ECs (PKH26); mice were tail injected with mouse FITC-lectin (green; host vessels);explants were analyzed by confocal microscopy; nuclei in blue with purple arrows indicate chimeric vessels; white arrows mark human vessels. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    The following description is provided to enable any person skilled in the art to make and use the invention and sets forth the best modes contemplated by the inventor of carrying out his invention. Various modifications, however, will remain readily apparent to those skilled in the art, since the general principles of the present invention have been defined herein specifically to provide a method for causing pluripotent stem cells to differentiate into effective ECs. 
         [0015]    The present inventors describe a step-wise protocol for differentiating mammalian, including human, pluripotent stem cells (PSCs) into ECs in vitro. The PSCs can be derived from any suitable source. For example, they can be embryonic stem cells (ESCs) or induced pluripotent stem cells (abbreviated iPS cells or iPSCs). The method, which is simple, efficient and reliable, allows for the efficient derivation of concentrated, purified, ECs. The derived ECs highly express specific endothelial cell markers. In the presence of extracellular matrix (Matrigel) the ECs mature to form linear and tubular structures. When transplanted into mammals these structures mature into actual vessels which can serve as a ready source for therapeutic vascular tissue engineering. 
         [0016]    As used herein, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. 
         [0017]    Throughout this application, the term “about” is used to mean plus or minus 10% of the value. For example, about 2×10 4  cells includes 1.8×10 4 -2.2×104 cells. Ranges as used herein include the endpoints of the range. 
         [0018]    “Pluripotent” cells, as used herein, refers to stem cells that have the potential to differentiate into any of the three germ layers: endoderm (interior stomach lining, gastrointestinal tract, the lungs), mesoderm (muscle, bone, blood, urogenital), or ectoderm (epidermal tissues and nervous system). Pluripotent stem cells can give rise to any fetal or adult cell type. 
         [0019]    Induced pluripotent cells (commonly abbreviated as iPS cells or iPSCs) are a type of pluripotent stem cell that is artificially derived from a non-pluripotent cell, such as an adult somatic cell, by forced expression of certain genes. Methods for generating iPS cells are conventional and well-known to those of skill in the art. 
         [0020]    Embryonic stem cells (ESCs) are described as “undifferentiated” when a substantial portion of stem cells and their derivatives in the population display morphological characteristics of undifferentiated cells, clearly distinguishing them from differentiated cells of embryonic or adult origin. Undifferentiated ES cells are easily recognized by those skilled in the art, and typically appear in a microscopic view as cells with high nuclear/cytoplasm ratios and prominent nucleoli. Similarly, undifferentiated cells can be distinguished from differentiated cells by the absence of lineage specific markers such as vascular endothelial growth factor receptor 2 (VEGFR2), vascular endothelial cadherin (VE-cad) or platelet-endothelial cell adhesion molecule-1 (PECAM-1). Often, hESCs are cultured with mouse embryonic fibroblasts (MEFs), a layer of feeder cells that nurture the hESCs and keep them in undifferentiated state. 
         [0021]    Much of the discussion in the present application is directed to iPSCs. However, other forms of PSCs, such as ESCs, are included. In a method of the invention, in the first culture step, individual undifferentiated ES cells are cultured in a manner suitable for inducing differentiation into vasculogenic progenitor cells. 
         [0022]    Before being plated and cultured, the PSCs, which often have been grown on a feeder layer, are treated with a suitable reagent (e.g., digested with trypsin, such as TrypLE, or treated with EDTA) to detach them from the culture plate, and are treated further to generate a single-cell suspension of cells that are smaller than about 50 μm (e.g., about 40 μm or smaller). The sizing step not only sorts the cells into cells of a desired size, but also separates them from undesirable, larger cells, such as feeder layer cells (e.g., MEFs) or EPC that may be present in the culture. Sizing methods such as filtration can also help to break up cells that have adhered to one another, e.g., in ESC colonies. 
         [0023]    Differentiation of individual undifferentiated PSCs can be affected by culturing such cells on plates coated with an adhesive substrate such as type IV collagen, laminin or gelatin to prevent aggregation of the ES cells; seeding the cells at a low plating density (at a seeding concentration of about 5×10 4  cells/cm 2 -about 1×10 5  cells/cm 2 , for example about 5×10  4  cells/cm 2 -about 7×10 4  cells/cm 2 , or about 5×10 4  cells/cm 2 ); and providing differentiation medium that contains no growth factors. In one embodiment, individual undifferentiated ES cells are grown on type IV collagen-coated plates (available from, for example, Cell Cultureware, BD-Falcon, Boston, Mass.). 
         [0024]    In the inventive method, hPSCs were cultured at a concentration of 1.25×10 4  per cm 2  on collagen IV plates for 12 days. In the “direct” method, the cells were treated with VEGF for the entire time. With the “sequential” method, the cells were supplemented with 10% serum and were not treated with VEGF until day 6. See  FIG. 1  for a comparison of the protocols. 
         [0025]    In our sequential method, hPSCs were cultured on collagen IV plates in media supplemented with 10% serum. After 6 days in culture, cells were strained and re-cultured in media supplemented with 50 ng/ml vascular endothelial growth factor (VEGF). Cells which were differentiated via direct means were cultured in the VEGF-supplemented media for all 12 days. EC markers were examined via immunofluorescence microscopy and flow cytometry. We tested hESC line, H9, as well as hiPSC line, MR31, which was derived from normal, fetal fibroblasts using three factors, Oct-4, Sox2, Klf4 (8, 9). 
         [0026]    MR31 cells differentiated via the sequential scheme yielded a greater percentage of vascular endothelial cadherin positive (VEcad+) cells (˜5%) compared to the direct method (˜2%). Expression of von Willebrand factor (vWF), which is a large glycoprotein characteristic of ECs, was observed in its characteristic speckled appearance in the differentiated cells. Using quantitative image analysis, we determine that the vWF expression was significantly up-regulated via the sequential differentiation method (˜5 fold). Moreover, culture on Matrigel (BD Biosciences) revealed that cells derived via the sequential method form more homogeneous tube-like branching structures, whereas the direct differentiation method primarily yielded cells that cluster together. Similar results were obtained when the comparative analyses were performed on differentiating H9 cells. Collectively, these results indicate a sequential differentiation scheme, compared to a direct one, is more conducive to endothelial differentiation. 
         [0027]    However, because sequentially-derived cells were only ˜5% VEcad+, we sought to improve upon the differentiation efficiency via biochemical means. To this end, we studied the addition of a TGFβ inhibitor, SB431542 (10) as well as angiogenic growth factors, bone morphogenetic protein-4 (BMP4) and Indian Hedgehog (Ihh). For our TGFβ inhibitor studies, we examined the effects of SB431542 in the second half of differentiation plus high (50 ng/ml) or low (1 ng/ml) VEGF. When we added SB431542 to the sequential differentiation scheme, we observed a dramatic increase in expression of mature EC marker, VEcad ( FIG. 2A ). Remarkably, VEcad expression increased regardless of high or low VEGF concentrations. Expression of CD31 and KDR were also increased in the presence of SB431542 ( FIG. 2A ). Cells expressed approximately 1% Tra-1-60 indicating they were fully differentiated. However, BMP4 or Ihh were not able to augment endothelial differentiation capacity above levels seen with SB431542 supplementation. 
         [0028]    We were interested in determining whether SMCs may be present in the derived population. Quantitative RT-PCR analysis revealed that SMC markers—smooth muscle myosin heavy chain and calponin—decreased to levels less than 0.05 (relative to day 0 cells) after 12 days, suggesting little to no SMC presence in our cultures ( FIG. 2B ). To establish a complete marker profile of the derived cells, we examined co-expression of VEcad with EC markers—CD31, CD105 and CD146—and with pericyte markers—CD73 and NG2. We observed that a subset of our cells were CD31+VEcad+ (˜8%;  FIG. 2C ). Our derived cells were enriched in CD105, CD146, and CD73 ( FIG. 2C ), but NG2 was not detected (data not shown). Further inquiry into the kinetics of marker expression along the course of differentiation demonstrated that expression of EC markers, VEcad, CD31, and KDR increased from day 6 to 12. Sub-culture of day 12 cells for an additional 6 days yielded further enrichment of VEcad, vWF, CD31, and lectin which were all properly localized ( FIG. 3A ,  3 B). 
         [0029]    After optimizing our 2D differentiation scheme, we investigated whether the derived ECs exhibited characteristic endothelial functionalities. Mature ECs are known for their ability to uptake acetylated low density lipoprotein (AcLDL) as part of LDL metabolism in the body and their capacity to commence vasculogenesis upon culture on Matrigel. Upon incubation with labeled AcLDL, a fraction of the derived cells exhibited internalized AcLDL ( FIG. 3C ). When our derived ECs were cultured on Matrigel, we observed cord-like structures after 24 hours ( FIG. 3D ). 
         [0030]    In vivo functionality is crucial to the success of derived ECs toward tissue engineering purposes. To ensure our derived ECs were able to survive implantation, form vascular networks, integrate with the host vasculature, and establish blood flow, the cells were encapsulated in Matrigel and subcutaneously implanted in nude mice. Derived ECs were labeled with PKH-26 (11) for ease of visualization in explanted tissue. To visualize angiogenesis in the implants prior to sample removal, mice were injected intravenously with anti-mouse FITC-lectin to enable visualization of mouse blood vessels within red-labeled (PKH) human vasculature using fluorescent microscopy. After 1 week, implants were harvested and imaged via confocal microscopy.  FIG. 4  shows the results; the DAPI image shows the cell nuclei; the lectin image shows the FITC labeled mouse tissue while the PKH image shows the human tissue. The “Merge” image shows the other 3 images overlapped. We found that our derived ECs were not only able to incorporate into the mouse vasculature forming chimeric vessels ( FIG. 4 , purple [upper] arrows), but cells also formed their own human vessels, devoid of mouse cells ( FIG. 4 , white [lower] arrows). 
         [0031]    Collectively, these findings reveal that hPSCs treated according to our method are able to differentiate into ECs which exhibit the appropriate marker expression profiles and are functional both in vitro and in vivo. 
         [0032]    Thus, we have developed chemically defined conditions for the controlled differentiation and robust derivation of functional ECs from both embryonic and induced pluripotent stem cells. The derived cells exhibit important functional characteristics of ECs such as uptake of AcLDL, cord formation on Matrigel, and incorporation in vivo. Because our cells are derived via controlled conditions in a 2D manner, these studies establish an efficient and clinically relevant methodology for deriving functional ECs. This invention will have considerable clinical impact with respect to improved vascular therapeutics and regenerative medicine. 
         [0033]    The following claims are thus to be understood to include what is specifically illustrated and described above, what is conceptually equivalent, what can be obviously substituted and also what incorporates the essential idea of the invention. The illustrated embodiment has been set forth only for the purposes of example and that should not be taken as limiting the invention. Therefore, it is to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein. 
         [0034]    References (incorporated herein by reference to the extent permissible by applicable laws and rules):
   1. S. I. Nishikawa, et al., Development, 125, 1747-1757 (1998).   2. S. Levenberg, et al., Blood, 110, 806-814 (2007).   3. S. Levenberg, et al., PNAS, 99, 4391-4396 (2002).   4. L. S. Ferreira, et al., Circulation Research, 101, 286-294 (2007).   5. M. A. Vodyanik and I. I. Slukvin, Current protocols in cell biology, Chapter 23 (2007).   6. K. L. Hill, et al., Exp Hematol, 38, 246-257 e241.   7. S. Gerecht-Nir, et al., Laboratory Investigation, 83, 1811-1820 (2003).   8. P. Mali, et al., Stem Cells, 28, 713-720 (2010).   9. A. Swistowski, et al., Stem Cells, 28, 1893-1904 (2010).   10. D. James, et al., Nature Biotechnology, 28, 161-166 (2010).   11. J. W. Ford et al., J. Surg. Res., 62, 23-28 (1996).