Human adipose-derived stem cells support the growth of limbal stem/progenitor cells.

The most efficient method to expand limbal stem cells (LSCs) in vitro for clinical transplantation is to culture single LSCs directly on growth-arrested mouse fibroblast 3T3 cells. To reduce possible xenobiotic contamination from 3T3s, primary human adipose-derived stem cells (ASCs) were examined as feeder cells to support the expansion of LSCs in vitro. To optimize the ASC-supported culture, freshly isolated limbal epithelial cells in the form of single cells (SC-ASC) or cell clusters (CC-ASC) were cultured using three different methods: LSCs seeded directly on feeder cells, a 3-dimensional (3D) culture system and a 3D culture system with fibrin (fibrin 3D). The expanded LSCs were examined at the end of a 2-week culture. The standard 3T3 culture served as control. Expansion of SC-ASC showed limited proliferation and exhibited differentiated morphology. CC-ASC generated epithelial cells with undifferentiated morphology in all culture methods, among which CC-ASC in 3D culture supported the highest cell doubling (cells doubled 9.0 times compared to cells doubled 4.9 times in control) while maintained the percentage of putative limbal stem/progenitor cells compared to the control. There were few cell-cell contacts between cultured LSCs and ASCs in 3D CC-ASC. In conclusion, ASCs support the growth of LSCs in the form of cell clusters but not in single cells. 3D CC-ASC could serve as a substitute for the standard 3T3 culture to expand LSCs.

A 3D culture system enhances the ability of human bone marrow stromal cells to support the growth of limbal stem/progenitor cells.

The standard method of cultivating limbal epithelial progenitor/stem cells (LSCs) on a monolayer of mouse 3T3 feeder cells possesses the risk of cross-contamination in clinical applications. Human feeder cells have been used to eliminate this risk; however, efficiency from xenobiotic-free cultures on a monolayer appears to be lower than in the standard method using 3T3 cells. We investigated whether bone marrow stromal cells (BMSCs), also known as bone marrow-derived mesenchymal stem cells, could serve as feeder cells for the expansion of LSCs in the 3-dimensional (3D) system. Primary single human LSCs on a monolayer of 3T3s served as the control. Very poor growth was observed when single LSCs were cultured on BMSCs. When LSC clusters were cultured on a BMSC monolayer (CC-BM), 3D culture system (3D CC-BM) and fibrin 3D system (fibrin 3D CC-BM), the 3D CC-BM method supported a greater LSC expansion. The 3D CC-BM system produced a 2.5-fold higher cell growth rate than the control (p<0.05). The proportion of K14(+) and p63α(bright) cells was comparable to those in the control (p>0.05), whereas the proportion of K12(+) cells was lower (p<0.05). These results indicate that BMSCs can efficiently support the expansion of the LSC population in the 3D culture.

Human limbal mesenchymal cells support the growth of human corneal epithelial stem/progenitor cells.

PURPOSE: We tested the viability of human limbal mesenchymal cells (LMCs) to support the expansion of human corneal epithelial stem/progenitor cells (LSCs). METHODS: Human LMCs were isolated from sclerocorneal tissue using collagenase A. Primary limbal epithelial cells (LECs) in the form of single cell suspension or cell clusters were cocultured on a monolayer of either 3T3 cells (control) or LMCs (SC-LMC culture). The LEC clusters also were grown directly on LMCs (CC-LMC culture) and in an optimized 3-dimensional culture method (3D CC-LMC culture). Colony-forming efficiency (CFE) and LEC proliferation were analyzed. The phenotype of the cultured LECs was assessed by their expression level of putative stem cell markers and a differentiation marker by qRT-PCR and immunocytochemistry. RESULTS: The LECs in the SC-LMC culture had a very limited growth and the stem/progenitor phenotype was lost compared to the control. Growth and cell morphology improved using the CC-LMC culture. The 3D CC-LMC culture method was the best to support the growth of the LSC population. Expression of ATP-binding cassette family G2 and ΔNp63 at the mRNA level was maintained or increased in CC-LMCs and 3D CC-LMC cultures compared to the control. The percentage of the K14(+) and K12(+) cells was comparable in these three cultures. There was no significant difference in the percentage of p63α high expressing cells in the control (21%) and 3D CC-LMC culture (17%, P > 0.05). CONCLUSIONS: Human LMCs can substitute 3T3 cells in the expansion of LSCs using the 3-dimensional culture system.

Frizzled 7 maintains the undifferentiated state of human limbal stem/progenitor cells.

Wnt signaling pathway plays an important role in the regulation of human limbal stem/progenitor cells (LSCs). To examine the possible function of Frizzled (Fz) receptors in LSCs, the expression of 10 Fz receptors was profiled in the limbus and cornea. Only Fz7 had preferential expression in the basal limbal epithelium which contains the LSCs. The expression of Fz7 was colocalized with the putative LSC markers including p63α, N-cadherin and keratin (K) 14, and was minimum in cells expressing the corneal maturation marker K12. The expression of Fz7 was higher in the enriched LSCs population and decreased in cultured LSCs when there was a loss of progenitor phenotype. When the Fz7 was knocked down (Fz(KD)) using shRNA in primary LSCs, the expression of putative LSC markers ABCG2, ΔNp63α, and K14 was decreased significantly. The colony forming efficiency of the Fz7(KD) LSCs was significantly decreased in the subsequent passage 1 and 2 compared to the control. Our finding suggests that Wnt signaling is one of the factors of LSC niche, and Fz7 helps to maintain the undifferentiated state of LSCs.

A three-dimensional culture method to expand limbal stem/progenitor cells.

The current standard method to culture human limbal stem/progenitor cells (LSCs) in vitro is to culture limbal epithelial cells directly on a layer of murine 3T3 feeder cells (standard method). The direct contact between human cells and murine feeder cells poses the potential risk of incomplete removal of feeder cells after culture and cross-contamination in clinical applications. We present here a novel three-dimensional (3D) sandwich method in which LSCs and feeder cells were separately cultured on opposite sides of a porous membrane. Limbal epithelial cells in the form of single-cell suspensions, cell clusters, and tissue explants were subjected to standard culture or to a 3D sandwich culture method. The 3D sandwich method consistently yielded LSCs derived from cell clusters and tissue explants. The expanded LSCs exhibited a small, compact, cuboidal stem-cell morphology and stem cell phenotypes comparable to those of LSCs derived from the standard culture method. Limbal epithelial cell clusters cultured with the sandwich method had a significantly higher proliferation rate than did those cultured with the standard method. The 3D sandwich method did not favor the propagation of single LSCs. In summary, the 3D sandwich method permits complete separation between cultured cells and feeder cells, while providing an even and maximal proximity between them. This alternative method permits culturing of LSCs without the risk of feeder cell contamination.

Enrichment of human corneal epithelial stem/progenitor cells by magnetic bead sorting using SSEA4 as a negative marker.

The use of a specific antibody bound to magnetic beads to isolate subpopulations of cells is an efficient and simple technique that allows for the subsequent study of different cell populations. One important use of this isolation technique is the purification of stem cells from a mixed cell population. In this protocol, we describe a method to purify human corneal epithelial stem/progenitor cells or limbal stem cells (LSC), using stage-specific embryonic antigen-4 (SSEA4) as a negative surface marker.

Identification of novel molecular markers through transcriptomic analysis in human fetal and adult corneal endothelial cells.

The corneal endothelium is composed of a monolayer of corneal endothelial cells (CECs), which is essential for maintaining corneal transparency. To better characterize CECs in different developmental stages, we profiled mRNA transcriptomes in human fetal and adult corneal endothelium with the goal to identify novel molecular markers in these cells. By comparing CECs with 12 other tissue types, we identified 245 and 284 signature genes that are highly expressed in fetal and adult CECs, respectively. Functionally, these genes are enriched in pathways characteristic of CECs, including inorganic anion transmembrane transporter, extracellular matrix structural constituent and cyclin-dependent protein kinase inhibitor activity. Importantly, several of these genes are disease target genes in hereditary corneal dystrophies, consistent with their functional significance in CEC physiology. We also identified stage-specific markers associated with CEC development, such as specific members in the transforming growth factor beta and Wnt signaling pathways only expressed in fetal, but not in adult CECs. Lastly, by the immunohistochemistry of ocular tissues, we demonstrated the unique protein localization for Wnt5a, S100A4, S100A6 and IER3, the four novel markers for fetal and adult CECs. The identification of a new panel of stage-specific markers for CECs would be very useful for characterizing CECs derived from stem cells or ex vivo expansion for cell replacement therapy.

The requirement for fibroblasts in angiogenesis: fibroblast-derived matrix proteins are essential for endothelial cell lumen formation.

A role for fibroblasts in physiological and pathological angiogenesis is now well recognized; however, the precise mechanisms underlying their action have not been determined. Using an in vitro angiogenesis model in combination with a candidate gene approach, column chromatography, and mass spectrometry, we identify two classes of fibroblast-derived factors--one that supports vessel sprouting but not lumen formation, and one that promotes lumen formation. In the absence of fibroblasts a combination of angiopoietin-1, angiogenin, hepatocyte growth factor, transforming growth factor-α, and tumor necrosis factor drives robust endothelial cell (EC) sprouting; however, lumens fail to form. Subsequent addition of fibroblast-conditioned medium restores lumenogenesis. Using small interfering RNA-mediated knockdown, we show that five genes expressed in fibroblasts--collagen I, procollagen C endopeptidase enhancer 1, secreted protein acidic and rich in cysteine, transforming growth factor-ß-induced protein ig-h3, and insulin growth factor-binding protein 7--are necessary for lumen formation. Moreover, lumen formation can be rescued by addition of purified protein to knockdown cultures. Finally, using rheology, we demonstrate that the presence of these matricellular proteins results in significantly stiffer gels, which correlates with enhanced lumen formation. These findings highlight the critical role that fibroblast-derived extracellular matrix components play in EC lumen formation and provide potential insight into the role of fibroblasts in the tumor microenvironment.

Keratin 13 is a more specific marker of conjunctival epithelium than keratin 19.

PURPOSE: To evaluate the expression patterns of cytokeratin (K) 12, 13, and 19 in normal epithelium of the human ocular surface to determine whether K13 could be used as a marker for conjunctival epithelium. METHODS: Total RNA was isolated from the human conjunctiva and central cornea. Those transcripts that had threefolds or higher expression levels in the conjunctiva than the cornea were identified using microarray technique. Expression levels of three known signature genes and of two conjunctival genes, K13 and K19 were confirmed by using quantitative real-time PCR (qRT-PCR). Protein expression of K12, K13, and K19 was confirmed by immunostaining with specific antibodies on histologic sections of human sclerocornea that contained the conjunctiva, limbus, and cornea and on impression cytology (IC) specimens of the cornea and conjunctiva from normal donors. Double staining of K13/K12 and K19/K12 on histologic sections and IC specimens was performed. RESULTS: There were 337 transcripts that were preferentially expressed in the conjunctiva. K13 and K19 were among the top twenty transcripts in the conjunctiva and this preferential expression pattern of K13 and K19 was confirmed by qRT-PCR. Immunohistochemical studies showed that K13 was expressed at the posterior limbal epithelium and conjunctival epithelium but was totally absent in the cornea. K12 was expressed in the corneal and anterior limbal epithelia except for the basal layer and was absent from the conjunctiva. In contrast, K19 was detected in the corneal, limbal and conjunctival epithelia. Immunostaining of the IC specimens showed K12(+) epithelial cells in the corneal region, K13(+) cells in the conjunctival area, and K19(+) cells in the corneal and conjunctival specimens. Expression of K13 and K12 on the ocular surface was mutually exclusive on both the histologic and IC samples using double immunostaining. CONCLUSIONS: K13 is more specific to the conjunctival epithelial cells than K19 and potentially could be used as a marker to identify conjunctival epithelial cells in limbal stem cell deficiency.

SSEA4 is a potential negative marker for the enrichment of human corneal epithelial stem/progenitor cells.

PURPOSE: To examine the expression of stage-specific embryonic antigen-4 (SSEA4) in the epithelium of the human ocular surface and characterize SSEA4(+) and SSEA4(-) limbal epithelial cells. METHODS: SSEA4 expression in the human cornea and limbus was examined by RT-PCR and immunohistochemistry. SSEA4(+) and SSEA4(-) cells were then separated by using magnetic beads. The phenotypes of these two cell populations were evaluated on the basis of cell size, clonogenic assay, and expression of putative limbal stem cell (LSC) and corneal epithelial differentiation markers. RESULTS: SSEA4 was expressed in all layers of the corneal and anterior limbal epithelia. Discrete clusters of SSEA4(+) cells were present in the central and posterior limbal epithelia. SSEA4(+) cells accounted for an average of 40% of the total limbal epithelial cells. The SSEA4(-) population contained five times more small cells (≤11 µm in diameter) than did the SSEA4(+) population. The expression levels of the putative LSC markers ABCG2, ΔNp63α, and cytokeratin (K)14 were significantly higher in the SSEA4(-) population than in the SSEA4(+) population. The SSEA4(-) cells also expressed a significantly higher level of N-cadherin, but a lower level of the differentiation marker K12. The colony-forming efficiency in the SSEA4(-) population was 25.2% (P = 0.04) and 1.6-fold (P < 0.05) higher than in the unsorted population and the SSEA4(+) population, respectively. CONCLUSIONS: SSEA4 is highly expressed in differentiated corneal epithelial cells, and SSEA4(-) limbal epithelial cells contain a higher proportion of limbal stem/progenitor cells. SSEA4 could be used as a negative marker to enrich the isolation of LSCs.

Wnt/ß-catenin signaling regulates proliferation of human cornea epithelial stem/progenitor cells.

PURPOSE: To investigate the expression and role of the Wnt signaling pathway in human limbal stem cells (LSCs). METHODS: Total RNA was isolated from the human limbus and central cornea. Limbal or cornea-specific transcripts were identified through quantitative real-time PCR. Protein expression of Wnt molecules was confirmed by immunohistochemistry on human ocular tissue. Activation of Wnt signaling using lithium chloride was achieved in vitro and its effects on LSC differentiation and proliferation were evaluated. RESULTS: Expression of Wnt2, Wnt6, Wnt11, Wnt16b, and four Wnt inhibitors were specific to the limbal region, whereas Wnt3, Wnt7a, Wnt7b, and Wnt10a were upregulated in the central cornea. Nuclear localization of ß-catenin was observed in a very small subset of basal epithelial cells only at the limbus. Activation of Wnt/ß-catenin signaling increased the proliferation and colony-forming efficiency of primary human LSCs. The stem cell phenotype was maintained, as shown by higher expression levels of putative corneal epithelial stem cell markers, ATP-binding cassette family G2 and ΔNp63α, and low expression levels of mature cornea epithelial cell marker, cytokeratin 12. CONCLUSIONS: These findings demonstrate for the first time that Wnt signaling is present in the ocular surface epithelium and plays an important role in the regulation of LSC proliferation. Modulation of Wnt signaling could be of clinical application to increase the efficiency of ex vivo expansion of corneal epithelial stem/progenitor cells for transplantation.

An optimized three-dimensional in vitro model for the analysis of angiogenesis.

Angiogenesis is the formation of new blood vessels from the existing vasculature. It is a multistage process in which activated endothelial cells (EC) degrade basement membrane, sprout from the parent vessel, migrate, proliferate, align, undergo tube formation, and eventually branch and anastomose with adjacent vessels. Here we describe a three-dimensional in vitro assay that reproduces each of these steps. Human umbilical vein endothelial cells (HUVEC) are cultured on microcarrier beads, which are then embedded in a fibrin gel. Fibroblasts cultured on top of the gel provide factors that synergize with bFGF and VEGF to promote optimal sprouting and tube formation. Sprouts appear around day 2, lumen formation begins at day 4, and at day 10 an extensive anastomosing network of capillary-like tubes is established. The EC express a similar complement of genes as angiogenic EC in vivo and undergo identical morphologic changes during tube formation. This model, therefore, recapitulates in vivo angiogenesis in several critical aspects and provides a system that is easy to manipulate genetically, can be visualized in real time, and allows for easy purification of angiogenic EC for downstream analysis.

TNF primes endothelial cells for angiogenic sprouting by inducing a tip cell phenotype.

Pathological angiogenesis associated with wound healing often occurs subsequent to an inflammatory response that includes the secretion of cytokines such as tumor necrosis factor (TNF). Controversy exists on the angiogenic actions of TNF, with it being generally proangiogenic in vivo, but antiangiogenic in vitro. We find that whereas continuous administration of TNF in vitro or in vivo inhibits angiogenic sprouting, a 2- to 3-day pulse stimulates angiogenesis by inducing an endothelial "tip cell" phenotype. TNF induces the known tip cell genes platelet-derived growth factor B (PDGFB) and vascular endothelial cell growth factor receptor-2 (VEGFR2), while at the same time blocking signaling through VEGFR2, thus delaying the VEGF-driven angiogenic response. Notch signaling regulates tip cell function, and we find that TNF also induces the notch ligand jagged-1, through an NFkappaB-dependent mechanism. Enrichment of jagged-1 in tip cells was confirmed by immunofluorescent staining as well as by laser capture microdissection/quantitative reverse-transcription-polymerase chain reaction (qRT-PCR) of tip cells sprouting in vitro. Thus, in angiogenesis, the temporal expression of TNF is critical: it delays angiogenesis initially by blocking signaling through VEGFR2, but in addition by inducing a tip cell phenotype through an NFkappaB-dependent pathway, it concomitantly primes endothelial cells (ECs) for sprouting once the initial inflammatory wave has passed.

Cell-autonomous notch signaling regulates endothelial cell branching and proliferation during vascular tubulogenesis.

The requirement for notch signaling during vascular development is well-documented but poorly understood. Embryonic and adult endothelial cells (EC) express notch and notch ligands; however, the necessity for cell-autonomous notch signaling during angiogenesis has not been determined. During angiogenesis, EC display plasticity, whereby a subset of previously quiescent cells loses polarity and becomes migratory. To investigate the role of notch in EC, we have used a three-dimensional in vitro system that models all of the early steps of angiogenesis. We find that newly forming sprouts are composed of specialized tip cells that guide the sprout and trunk cells that proliferate and rearrange to form intercellular lumens. Furthermore, we find that notch acts cell-autonomously to suppress EC proliferation, thereby regulating tube diameter. In addition, when notch signaling is blocked, tip cells divide, and both daughter cells take on a tip cell phenotype, resulting in increased branching through vessel bifurcation. In contrast, notch signaling is not required for re-establishment of EC polarity or for lumen formation. Thus, notch is used reiteratively and cell-autonomously by EC to regulate vessel diameter, to limit branching at the tip of sprouts, and to establish a mature, quiescent phenotype.

VEGF(121) and VEGF(165) regulate blood vessel diameter through vascular endothelial growth factor receptor 2 in an in vitro angiogenesis model.

Vascular endothelial growth factor (VEGF) is essential for the induction of angiogenesis and drives both endothelial cell (EC) proliferation and migration. It has been suggested that VEGF also regulates vessel diameter, although this has not been tested explicitly. The two most abundant isoforms, VEGF(121) and VEGF(165), both signal through VEGF receptor 2 (VEGFR-2). We recently optimized a three-dimensional in vitro angiogenesis assay using HUVECs growing on Cytodex beads and embedded in fibrin gels. Fibroblasts provide critical factors that promote sprouting, lumen formation, and vessel stability. Using this assay, we have examined the role of VEGF in setting vessel diameter. Low concentrations of both VEGF(121) and VEGF(165) promote growth of long, thin vessels, whereas higher concentrations of VEGF remarkably enhance vessel diameter. Placental growth factor, which binds to VEGFR-1 but not VEGFR-2, does not promote capillary sprouting. Moreover, specific inhibition of VEGFR-2 signaling results in a dramatic reduction of EC sprouting in response to VEGF, indicating the critical importance of this receptor. The increase in vessel diameter is the result of cell proliferation and migration, rather than cellular hypertrophy, and likely depends on MEK1-ERK1/2 signaling. Both phosphatidylinositol 3-kinase and p38 activity are required for cell survival. We conclude that the diameter of new capillary sprouts can be determined by the local concentration of VEGF and that the action of VEGF on angiogenic EC in this assay is critically dependent on signaling through VEGFR-2.

Angiogenic sprouting and capillary lumen formation modeled by human umbilical vein endothelial cells (HUVEC) in fibrin gels: the role of fibroblasts and Angiopoietin-1.

Angiogenesis is a multistep process of critical importance both in development and in physiological and pathophysiological processes in the adult. It involves endothelial cell (EC) sprouting from the parent vessel, followed by migration, proliferation, alignment, tube formation, and anastomosis to other vessels. Several in vitro models have attempted to recreate this complex sequence of events with varying degrees of success. We report an optimized protocol for human umbilical vein EC in which EC sprout from the surface of beads embedded in fibrin gels. Fibroblast-derived factors, other than Angiopoietin-1, promote sprouting, lumen formation, and long-term stability of neovessels. Analysis by time-lapse and still photomicroscopy demonstrates dynamic vessels guided by a "tip cell" that extends numerous processes into the gel. Behind this cell a lumen forms, surrounded by a single layer of polarized EC. The growing sprouts express notch 1, notch 4, and delta 4, as well as the downstream notch effector HESR-1. Importantly, cells can be infected with adenovirus to high efficiency without compromising sprout formation, thus allowing for manipulation of gene expression. This improved model recapitulates all the major steps of angiogenesis seen in vivo and provides a powerful model for analysis of this complex phenomenon.

Identification of endothelial cell genes expressed in an in vitro model of angiogenesis: induction of ESM-1, (beta)ig-h3, and NrCAM.

Blood vessel growth by angiogenesis plays an essential role in embryonic development, wound healing, and tumor growth. To understand the molecular cues underlying this process we have used the PCR-based subtractive hybridization method, representational difference analysis, to identify genes upregulated in endothelial cells (EC) forming tubes in 3D collagen gels, compared to migrating and proliferating cells in 2D cultures. We identified several previously characterized angiogenic markers, including the alpha(v) chain of the alpha(v)beta3 integrin and plasminogen activator inhibitor-1, suggesting overlap in gene expression between tube-forming cells in vitro and in vivo. We also found a 2- to 10-fold upregulation of (beta)ig-h3 (a collagen-binding extracellular matrix protein), NrCAM (a "neural" cell adhesion molecule), Annexin II (a tPA receptor), ESM-1 (an EC-specific molecule of unknown function), and Id2 (an inhibitory bHLH transcription factor). We identified a novel splice variant of the ESM-1 gene and also detected dramatically enhanced expression of ESM-1 and (beta)ig-h3 in several tumors. Antisense oligonucleotides to (beta)ig-h3 blocked both gene expression and tube formation in vitro, suggesting that (beta)ig-h3 may play a critical role in EC-matrix interactions. These data expand the suite of genes implicated in vascular remodeling and angiogenesis.