Comparison between serum-free and fibroblast-cocultured single-cell clonal culture systems: evidence showing that epithelial anti-apoptotic activity is present in 3T3 fibroblast-conditioned media

The Ins and Outs of Clusterin: Its Role in Cancer, Eye Diseases and Wound Healing

Clusterin (CLU) is a glycoprotein originally discovered in 1983 in ram testis fluid. Rapidly observed in other tissues, it was initially given various names based on its function in different tissues. In 1992, it was finally named CLU by consensus. Nearly omnipresent in human tissues, CLU is strongly expressed at fluid-tissue interfaces, including in the eye and in particular the cornea. Recent research has identified different forms of CLU, with the most prominent being a 75-80 kDa heterodimeric protein that is secreted. Another truncated version of CLU (55 kDa) is localized to the nucleus and exerts pro-apoptotic activities. CLU has been reported to be involved in various physiological processes such as sperm maturation, lipid transportation, complement inhibition and chaperone activity. CLU was also reported to exert important functions in tissue remodeling, cell-cell adhesion, cell-substratum interaction, cytoprotection, apoptotic cell death, cell proliferation and migration. Hence, this protein is sparking interest in tissue wound healing. Moreover, CLU gene expression is finely regulated by cytokines, growth factors and stress-inducing agents, leading to abnormally elevated levels of CLU in many states of cellular disturbance, including cancer and neurodegenerative conditions. In the eye, CLU expression has been reported as being severely increased in several pathologies, such as age-related macular degeneration and Fuch's corneal dystrophy, while it is depleted in others, such as pathologic keratinization. Nevertheless, the precise role of CLU in the development of ocular pathologies has yet to be deciphered. The question of whether CLU expression is influenced by these disorders or contributes to them remains open. In this article, we review the actual knowledge about CLU at both the protein and gene expression level in wound healing, and explore the possibility that CLU is a key factor in cancer and eye diseases. Understanding the expression and regulation of CLU could lead to the development of novel therapeutics for promoting wound healing.

Cultivated Autologous Limbal Epithelial Cell Transplantation: New Frontier in the Treatment of Limbal Stem Cell Deficiency

PURPOSE

Taking into consideration prior human experience with treating limbal stem cell deficiency (LSCD) with cultivated limbal epithelial cells (CLEC) from other countries, we have set a goal to optimize and standardize the techniques of CLEC preparation (called CALEC by our group) for the clinical trial in the United States.

METHODS

We performed an extensive literature review of all human trials, case series, and reports involving autologous cultivated limbal epithelial cell transplantation. Allogeneic cultivated limbal epithelial cell transplantations were reported only when combined with autologous studies. We also searched prior animal data aiding in detailing regulatory toxicology requirements.

RESULTS

Between 1997 and 2020, the analysis of human trials revealed 21 studies on autologous grafts, and 13 studies analyzing both autologous grafts and allogeneic grafts. Of a total of 34 studies, 6 studies used good manufacturing process (GMP) facilities, and 11 studies had no animal-derived products or murine feeder layers, whereas only 1 study had both. Overall, the treatment with autologous CLEC grafts was 68.9% successful. In total there were 6 preclinical studies using rabbits, serving as surrogate studies to assess the safety and toxicity of cultivated limbal epithelial cells for human trials. Based on prior human experience, we further optimized the manufacturing conditions with GMP-grade and serum and animal-free reagents, and developed cell characterization assays for the CALEC product release.

CONCLUSIONS

These data were used to develop a novel and consistent manufacturing process using only qualified and validated reagents for performing the first clinical trial on CALEC transplantation to treat LSCD in the United States.

Clusterin, other extracellular chaperones, and eye disease

Proteostasis refers to all the processes that maintain the correct expression level, location, folding and turnover of proteins, essential to organismal survival. Both inside cells and in body fluids, molecular chaperones play key roles in maintaining proteostasis. In this article, we focus on clusterin, the first-recognized extracellular mammalian chaperone, and its role in diseases of the eye. Clusterin binds to and inhibits the aggregation of proteins that are misfolded due to mutations or stresses, clears these aggregating proteins from extracellular spaces, and facilitates their degradation. Clusterin exhibits three main homeostatic activities: proteostasis, cytoprotection, and anti-inflammation. The so-called "protein misfolding diseases" are caused by aggregation of misfolded proteins that accumulate pathologically as deposits in tissues; we discuss several such diseases that occur in the eye. Clusterin is typically found in these deposits, which is interpreted to mean that its capacity as a molecular chaperone to maintain proteostasis is overwhelmed in the disease state. Nevertheless, the role of clusterin in diseases involving such deposits needs to be better defined before therapeutic approaches can be entertained. A more straightforward case can be made for therapeutic use of clusterin based on its proteostatic role as a proteinase inhibitor, as well as its cytoprotective and anti-inflammatory properties. It is likely that clusterin works together in this way with other extracellular chaperones to protect the eye from disease, and we discuss several examples. We end this article by predicting future steps that may lead to development of clusterin as a biological drug.

Novel Cell Culture Paradigm Prolongs Mouse Corneal Epithelial Cell Proliferative Activity in vitro and in vivo

It has been a long-standing challenge to obtain from cell cultures adequate amounts of mouse corneal epithelial cells (mCEC) to perform transplantation surgery. This limitation is attributable to the passage dependent declines in their proliferative activity. We describe here development of a novel 6C medium that contains six different modulators of different signaling pathways, which control proliferative mCEC activity. Its usage shortens the time and effort required to obtain epithelial sheets for hastening healing of an epithelial wound in an experimental animal model. This serum-free 6C medium contains:Y27632, forskolin, SB431542, DAPT, IWP-2, LDN-193189 and also DermaLife K keratinocyte calcium. Their inclusion inhibits rises in four specific markers of epithelial mesenchymal transdifferentiation:ZEB1/2, Snail, β-catenin and α-SMA. This medium is applied in a feeder-free air-lifted system to obtain sufficient populations of epithelial progenitor cells whose procurement is facilitated due to suppression of progenitor epithelial cell transdifferentiation into epithelial-mesenchymal cells. Diminution of this decline in transdifferentiation was confirmed based on the invariance of P63, K14, Pax6, and K12 gene expression levels. This cell culture technique is expected to facilitate ex vivo characterization of mechanisms underlying cell fate determination. Furthermore, its implementation will improve yields of progenitor mouse corneal epithelial cells, which increases the likelihood of using these cells as a source to generate epithelial sheets for performing transplantation surgery to treat limbal stem cell deficiency in a clinical setting. In addition, the novel insight obtainable from such studies is expected to improve the outcomes of corneal regenerative medicine.

The Human Tissue-Engineered Cornea (hTEC): Recent Progress

Each day, about 2000 U.S. workers have a job-related eye injury requiring medical treatment. Corneal diseases are the fifth cause of blindness worldwide. Most of these diseases can be cured using one form or another of corneal transplantation, which is the most successful transplantation in humans. In 2012, it was estimated that 12.7 million people were waiting for a corneal transplantation worldwide. Unfortunately, only 1 in 70 patients received a corneal graft that same year. In order to provide alternatives to the shortage of graftable corneas, considerable progress has been achieved in the development of living corneal substitutes produced by tissue engineering and designed to mimic their in vivo counterpart in terms of cell phenotype and tissue architecture. Most of these substitutes use synthetic biomaterials combined with immortalized cells, which makes them dissimilar from the native cornea. However, studies have emerged that describe the production of tridimensional (3D) tissue-engineered corneas using untransformed human corneal epithelial cells grown on a totally natural stroma synthesized by living corneal fibroblasts, that also show appropriate histology and expression of both extracellular matrix (ECM) components and integrins. This review highlights contributions from laboratories working on the production of human tissue-engineered corneas (hTECs) as future substitutes for grafting purposes. It overviews alternative models to the grafting of cadaveric corneas where cell organization is provided by the substrate, and then focuses on their 3D counterparts that are closer to the native human corneal architecture because of their tissue development and cell arrangement properties. These completely biological hTECs are therefore very promising as models that may help understand many aspects of the molecular and cellular mechanistic response of the cornea toward different types of diseases or wounds, as well as assist in the development of novel drugs that might be promising for therapeutic purposes.

Optimization and Characterization of PHA (SCL-SCL) Copolymer by Indigenous Bacillus thuringiensis A102 Strain for Biomedical Applications

PHA is one of the leading commercially important bio-polyesteric compounds piled up as an intracellular lipid-based energy storage compound by numerous microorganisms. An indigenous Gram-positive bacterium isolated from fire ant (Solenopsis invicta) has known to potentially accumulate PHA. Various nutritional elements like carbon, nitrogen, phosphate and C/N ratio were optimized. The indigenous B.t.A102 strain grown in optimized RC medium yielded PHA of about 3.25 g/L. The extracted polymer was characterized by NMR, GC-MS, X-ray diffraction and thermal analysis via TGA & DTA. The characterized PHA was used to prepare scaffold using the solvent casting method. The non-toxic nature of the composite material was evaluated on NIH/3T3 fibroblast cell lines using different staining (like Giemsa staining, AO/EB dual staining, neutral red staining) techniques and cell viability assay. This paper dealt with the optimization of the media components that increase PHA production and primary in vitro testing for its possible application as wound dressing materials.

Human corneal limbal organoids maintaining limbal stem cell niche function

Corneal epithelial stem cells reside in the limbal area between the cornea and conjunctiva. We examined the potential use of limbal organoids as a source of transplantable limbal stem cells. After treating tissue with collagenase, limbal cells were seeded onto Matrigel and cultivated using limbal phenotype maintenance medium. After 1-month, approximately 500 organoids were formed from one donor cornea. Organoids derived from vertical sites (superior and inferior limbus) showed large colony forming efficiency, a higher ratio of slow cycling cells and N-cadherin-expressing epithelial cells compared to horizontal sites. The progenitor markers Keratin (K) 15 and p63 were expressed in epithelial sheets engineered form a single organoid. Organoids transplanted in the limbus of a rabbit limbal deficiency model confirmed the presence of organoid-derived cells extending on to host corneas by immunohistochemistry. Our data show that limbal organoids with a limbal phenotype can be maintained for up to 1 month in vitro which can each give rise to a fully stratified corneal epithelium complete with basal progenitor cells. Limbal organoids were successfully engrafted in vivo to provide epithelial cells in a rabbit limbal deficiency model, suggesting that organoids may be an efficient cell source for clinical use.

Primary Culture of Cornea-Limbal Epithelial Cells In Vitro

The cultivation of corneal-limbal cells in vitro represents an excellent means to generate models to study cornea function and disease processes. These in vitro expanded cornea-limbal epithelial cell cultures are rich in stem cells for cornea, and hence can be used as a cell therapy for cornea-limbal deficiency. This chapter details the primary culture of these cornea-limbal cells, which can be used as model for further studies of the cornea surface.

Irradiated Human Fibroblasts as a Substitute Feeder Layer to Irradiated Mouse 3T3 for the Culture of Human Corneal Epithelial Cells: Impact on the Stability of the Transcription Factors Sp1 and NFI

Because of the worldwide shortage of graftable corneas, alternatives to restore visual impairments, such as the production of a functional human cornea by tissue engineering, have emerged. Self-renewal of the corneal epithelium through the maintenance of a sub-population of corneal stem cells is required to maintain the functionality of such a reconstructed cornea. We previously reported an association between stem cell differentiation and the level to which they express the transcription factors Sp1 and NFI. In this study, we investigated the impact of replacing irradiated 3T3 (i3T3) murine fibroblast feeder cells by irradiated human corneal fibroblasts (iHFL) on the expression of Sp1 and NFI and evaluated their contribution to the proliferative properties of human corneal epithelial cells (hCECs) in both monolayer cultures and human tissue engineered corneas (hTECs). hCECs co-cultured with iHFL could be maintained for up to two more passages than when they were grown with i3T3. Western Blot and electrophoretic mobility shift assays (EMSAs) revealed no significant difference in the feeder-layer dependent increase in Sp1 at both the protein and DNA binding level, respectively, between HCECs grown with either i3T3 or iHFL. On the other hand, a significant increase in the expression and DNA binding of NFI was observed at each subsequent passage when hCECs were co-cultured along with i3T3. These changes were found to result from an increased expression of the NFIA and NFIB isoforms in hCECs grown with i3T3. Exposure of hCECs to cycloheximide revealed an increased stability of NFIB that likely resulted from post-translational glycosylation of this protein when these cells were co-cultured with i3T3. In addition, iHFL were as efficient as i3T3 at preserving corneal, slow-cycling, epithelial stem cells in the basal epithelium of the reconstructed hTECs. Furthermore, we observed an increased expression of genes whose encoded products promote hCECs differentiation along several passages in hCECs co-cultured with either type of feeder layer. Therefore, the iHFL feeder layer appears to be the most effective at maintaining the proliferative properties of hCECs in culture most likely by preserving high levels of Sp1 and low levels of NFIB, which is known for its gene repressor and cell differentiation properties.

Transient Mitomycin C-treatment of human corneal epithelial cells and fibroblasts alters cell migration, cytokine secretion, and matrix accumulation

A single application of Mitomycin C (MMC) is used clinically in ophthalmology to reduce scarring and enhance wound resolution after surgery. Here we show in vitro that a 3-hour MMC treatment of primary and telomerase immortalized human corneal limbal epithelial (HCLE) cells impacts their migration and adhesion. Transient MMC treatment induces HCLE expression of senescence associated secretory factors, cytokine secretion, and deposition of laminin 332 for several days. Transient MMC treatment also reduces migration and deposition of transforming growth factor-β1 (TGFβ1)-stimulated collagen by corneal fibroblasts. Using conditioned media from control and MMC treated cells, we demonstrate that factors secreted by MMC-treated corneal epithelial cells attenuate collagen deposition by HCFs whereas those secreted by MMC-treated HCFs do not. These studies are the first to probe the roles played by corneal epithelial cells in reducing collagen deposition by corneal fibroblasts in response to MMC.

Establishment of Novel Limbus-Derived, Highly Proliferative ABCG2+/ABCB5+ Limbal Epithelial Stem Cell Cultures

Homeostasis and regeneration of corneal epithelia are sustained by limbal epithelial stem cells (LESCs); thus, an LESC deficiency is a major cause of blindness worldwide. Despite the generally promising results of cultivated LESC transplantation, it has been limited by variations in long-term success rates, the use of xenogeneic and undefined culture components, and a scarcity of donor tissues. In this study, we identified the culture conditions required to expand LESCs in vitro and established human limbus-derived highly proliferative ABCG2⁺/ABCB5⁺ double-positive LESCs. These LESCs exhibited the LESC marker profile and differentiated into corneal epithelial cells. In addition, cultured LESCs expressed high levels of the stem cell markers Sox2, Oct4, c-Myc, and Klf4, had high telomerase activity, and had stable, normal genomes. These results suggest that our novel cultivation protocol affects the phenotype and differentiation capacity of LESCs. From the limbus, which contains a heterogenous cell population, we have derived highly proliferative ABCG2⁺/ABCB5⁺ double-positive cells with the ability to differentiate into corneal epithelial cells. This study opens a new avenue for investigation of the molecular mechanism of LESC maintenance and expansion in vitro and may impact the treatment of corneal disease, particularly corneal blindness due to an LESC deficiency.

The characterization of human oral mucosal fibroblasts and their use as feeder cells in cultivated epithelial sheets

Aim

To characterize human oral mucosa middle interstitial tissue fibroblasts (hOMFs) and their application in the cultivation of epithelial sheets.

Methodology

hOMFs were cultured with methylcellulose to form cell clusters. hOMFs amplified in adhesive culture were analyzed by flow cytometry, and were found to differentiate into multiple cell types suitable for the cultivation of human corneal epithelial sheets. hOMFs were expanded from clusters to analyze CD56 and PDGFRα expression.

Results

These cells showed similar differentiation patterns as keratocytes, and similar expression patterns as mesenchymal and neural cells. Furthermore, we established human corneal epithelial sheets using hOMFs.

Conclusion

hOMFs may be of neural crest origin and possess multipotent differentiation capacity, and are suitable for use as an autologous cell source for corneal regeneration.

Comparison of the Application of Allogeneic Fibroblast and Autologous Mesh Grafting With the Conventional Method in the Treatment of Third-Degree Burns

Wound healing is a multipart process involving different cell types and growth factors. Third-degree burns are usually treated by early excision and skin grafting. Tissue engineering has been developed in this field in response to limitations associated with autografts. Allogeneic fibroblasts on meshed split thickness skin grafts (STSGs) are known to have useful properties in wound healing and can be used to construct a new model of living skin substitute. Fourteen patients were chosen from June 2009 until December 2010 as the sample for this study. After debridement and wound excision, meshed STSG was used to cover the entire wound. Alloskin (allofibroblasts cultured on a combination of silicone and glycosaminoglycan) was applied on one side and petroleum jelly-impregnated gauze (Iran Polymer and Petrochemical Institute) was applied on the other. The healing time, scar formation, and pigmentation score were assessed for the patients. All analyses were undertaken with SPSS 17 software. Alloskin demonstrated good properties compared to petroleum jelly-impregnated gauze. The average healing time and hypertrophic scar formation were significantly different between the two groups. In addition, the skin pigmentation score in the alloskin group was closer to normal. Alloskin grafting, including fibroblasts on meshed STSG, may be a useful method to reduce healing time and scar size and may require less autologous STSG in extensive burns where a high percentage of skin is burned and there is a lack of available donor sites.

Porous silk fibroin film as a transparent carrier for cultivated corneal epithelial sheets

Biological carriers, such as the amniotic membrane and serum-derived fibrin, are currently used to deliver cultivated corneal epithelial sheets to the ocular surface. Such carriers require being transparent and allowing the diffusion of metabolites in order to maintain a healthy ocular surface. However, safety issues concerning biological agents encouraged the development of safer, biocompatible materials as cell carriers. We examined the application of porous silk fibroin films with high molecular permeability prepared by mixing silk fibroin and poly(ethylene glycol) (PEG), and then removal of PEG from the silk-PEG films. Molecular permeability of porous silk fibroin film is higher than untreated silk fibroin film. Epithelial cells were isolated from rabbit limbal epithelium, and seeded onto silk fibroin coated wells and co-cultured with mitomycin C-treated 3T3 fibroblasts. Stratified epithelial sheets successfully engineered on porous silk fibroin film expressed the cornea-specific cytokeratins K3 and K12, as well as the corneal epithelial marker pax6. Basement membrane components such as type-IV collagen and integrin β1 were expressed in the stratified epithelial sheets. Further more, colony-forming efficiency of dissociated cells was similar to primary corneal epithelial cells showing that progenitor cells were preserved. The biocompatibility of fibroin films was confirmed in rabbit corneas for up to 6 months. Porous silk fibroin film is a highly transparent, biocompatible material that may be useful as a carrier of cultivated epithelial sheets in the regeneration of corneal epithelium.

Clusterin in the eye: An old dog with new tricks at the ocular surface

The multifunctional protein clusterin (CLU) was first described in 1983 as a secreted glycoprotein present in ram rete testis fluid that enhanced aggregation ('clustering') of a variety of cells in vitro. It was also independently discovered in a number of other systems. By the early 1990s, CLU was known under many names and its expression had been demonstrated throughout the body, including in the eye. Its homeostatic activities in proteostasis, cytoprotection, and anti-inflammation have been well documented, however its roles in health and disease are still not well understood. CLU is prominent at fluid-tissue interfaces, and in 1996 it was demonstrated to be the most highly expressed transcript in the human cornea, the protein product being localized to the apical layers of the mucosal epithelia of the cornea and conjunctiva. CLU protein is also present in human tears. Using a preclinical mouse model for desiccating stress that mimics human dry eye disease, the authors recently demonstrated that CLU prevents and ameliorates ocular surface barrier disruption by a remarkable sealing mechanism dependent on attainment of a critical all-or-none concentration in the tears. When the CLU level drops below the critical all-or-none threshold, the barrier becomes vulnerable to desiccating stress. CLU binds selectively to the ocular surface subjected to desiccating stress in vivo, and in vitro to LGALS3 (galectin-3), a key barrier component. Positioned in this way, CLU not only physically seals the ocular surface barrier, but it also protects the barrier cells and prevents further damage to barrier structure. CLU depletion from the ocular surface epithelia is seen in a variety of inflammatory conditions in humans and mice that lead to squamous metaplasia and a keratinized epithelium. This suggests that CLU might have a specific role in maintaining mucosal epithelial differentiation, an idea that can now be tested using the mouse model for desiccating stress. Most excitingly, the new findings suggest that CLU could serve as a novel biotherapeutic for dry eye disease.

Reconstruction of auto-tissue-engineered lamellar cornea by dynamic culture for transplantation: a rabbit model

To construct an auto-tissue-engineered lamellar cornea (ATELC) for transplantation, based on acellular porcine corneal stroma and autologous corneal limbal explants, a dynamic culture process, which composed of a submersion culture, a perfusion culture and a dynamic air-liquid interface culture, was performed using appropriate parameters. The results showed that the ATELC-Dynamic possessed histological structure and DNA content that were similar to native lamellar cornea (NLC, p>0.05). Compared to NLC, the protein contents of zonula occludens-1, desmocollin-2 and integrin β4 in ATELC-Dynamic reached 93%, 89% and 73%, respectively. The basal cells of ATELC-Dynamic showed a better differentiation phenotype (K3⁻, P63⁺, ABCG2⁺) compared with that of ATELC in static air-lift culture (ATELC-Static, K3⁺, P63⁻, ABCG2⁻). Accordingly, the cell-cloning efficiency of ATELC-Dynamic (9.72±3.5%) was significantly higher than that of ATELC-Static (2.13±1.46%, p<0.05). The levels of trans-epithelial electrical resistance, light transmittance and areal modulus variation in ATELC-Dynamic all reached those of NLC (p>0.05). Rabbit lamellar keratoplasty showed that the barrier function of ATELC-Dynamic was intact, and there were no signs of epithelial shedding or neovascularization. Furthermore, the ATELC-Dynamic group had similar optical properties and wound healing processes compared with the NLC group. Thus, the sequential dynamic culture process that was designed according to corneal physiological characteristics could successfully reconstruct an auto-lamellar cornea with favorable morphological characteristics and satisfactory physiological function.

From stem cell niche environments to engineering of corneal epithelium tissue

Studies on stem cells (SC) show that SC functions are determined by the extracellular microenvironment, known as the "niche", and by intrinsic genetic programs in the SCs; both are involved in regulating the delicate balance of self-renewal and differentiation. We have identified an animal model of limbal SC (LSC) deficiency and transplantation of SC-containing limbal tissue to treat the LSC deficiency, which could not only replace LSCs by providing new healthy corneal epithelial cells but also restore the lost niche of the limbal stromal layer, causing the regression of vessels and rearrangement of the corneal stromal lamellae. The purpose of the ex-vivo expansion technique is to develop a method that will enable culture of a small number of SCs which could than be expanded in a defined cultured system while preserving the original characteristics and properties of the SCs. In addition, SC characteristics will continue to be maintained when the cultured cells are transplanted back into the host. Bromodeoxyuridine-retaining, ΔNp63, ABCG2, p120, and N-cadherin immunoreactive studies of LSC cultured on an amniotic membrane have been performed. Pathological studies have been conducted for cases with preexisting central corneal stromal opacity treated by transplantation of LSCs followed by penetrating keratoplasty. The results indicate that the amniotic membrane can provide the niche environment for cultured LSCs and maintain the limbal-like environment for the transplanted area of cornea.

Limbal epithelial cell therapy: past, present, and future

The cornea, the clear window at the front of the eye, transmits light to the retina to enable vision. The corneal surface is renewed by stem cells located at the peripheral limbal region. These cells can be destroyed by a number of factors, including chemical burns, infections, and autoimmune diseases, which result in limbal stem cell deficiency (LSCD), a condition that can lead to blindness. Established therapy for LSCD based on ex vivo expanded limbal epithelial cells is currently at a stage of refinement. Therapy for LSCD is also rapidly evolving to include alternative cell types and clinical approaches as treatment modalities. In the present perspectives chapter, strategies to treat LSCD are discussed and advances in this important field of regenerative medicine are highlighted.

Identification of human fibroblast cell lines as a feeder layer for human corneal epithelial regeneration

There is a great interest in using epithelium generated in vitro for tissue bioengineering. Mouse 3T3 fibroblasts have been used as a feeder layer to cultivate human epithelia including corneal epithelial cells for more than 3 decades. To avoid the use of xeno-components, we evaluated human fibroblasts as an alternative feeder supporting human corneal epithelial regeneration. Five human fibroblast cell lines were used for evaluation with mouse 3T3 fibroblasts as a control. Human epithelial cells isolated from fresh corneal limbal tissue were seeded on these feeders. Colony forming efficiency (CFE) and cell growth capacity were evaluated on days 5–14. The phenotype of the regenerated epithelia was evaluated by morphology and immunostaining with epithelial markers. cDNA microarray was used to analyze the gene expression profile of the supportive human fibroblasts. Among 5 strains of human fibroblasts evaluated, two newborn foreskin fibroblast cell lines, Hs68 and CCD1112Sk, were identified to strongly support human corneal epithelial growth. Tested for 10 passages, these fibroblasts continually showed a comparative efficiency to the 3T3 feeder layer for CFE and growth capacity of human corneal epithelial cells. Limbal epithelial cells seeded at 1×10⁴ in a 35-mm dish (9.6 cm²) grew to confluence (about 1.87–2.41×10⁶ cells) in 12–14 days, representing 187–241 fold expansion with over 7–8 doublings on these human feeders. The regenerated epithelia expressed K3, K12, connexin 43, p63, EGFR and integrin β1, resembling the phenotype of human corneal epithelium. DNA microarray revealed 3 up-regulated and 10 down-regulated genes, which may be involved in the functions of human fibroblast feeders. These findings demonstrate that commercial human fibroblast cell lines support human corneal epithelial regeneration, and have potential use in tissue bioengineering for corneal reconstruction.

Ocular epithelial transplantation: current uses and future potential

Visual loss may be caused by a variety of ocular diseases and places a significant burden on society. Replacing or regenerating epithelial structures in the eye has been demonstrated to recover visual loss in a number of such diseases. Several types of cells (e.g., embryonic stem cells, adult stem/progenitor/differentiated epithelial cells and induced pluripotent cells) have generated much interest and research into their potential in restoring vision in a variety of conditions: from ocular surface disease to age-related macular degeneration. While there has been some success in clinical transplantation of conjunctival and particularly corneal epithelium utilizing ocular stem cells, in particular, from the limbus, the replacement of the retinal pigment epithelium by utilizing stem cell sources has yet to reach the clinic. Advances in our understanding of all of these cell types, their differentiation and subsequent optimization of culture conditions and development of suitable substrates for their transplantation will enable us to overcome current clinical obstacles. This article addresses the current status of knowledge concerning the biology of stem cells, their progeny and the use of differentiated epithelial cells to replace ocular epithelial cells. It will highlight the clinical outcomes to date and their potential for future clinical use.

Epidermal differentiation and loss of clonal growth potential of human limbal basal epithelial progenitor cells during intrastromal invasion

PURPOSE

Intrastromal invasion by limbal basal epithelial progenitor cells in explant cultures is associated with epithelial-mesenchymal transition. It remains unclear whether intrastromal invasion is contingent on culturing conditions and whether invaded cells retain their progenitor status and original lineage.

METHODS

Human limbal explants were cultured on various culture substrates, with or without air-lifting (AL), and subjected to hematoxylin and eosin staining and immunostaining to pan-cytokeratins, p63α, ΔNp63, Pax6, CK10, and CK12. Single cells obtained by trypsin/EDTA from dispase-isolated epithelial sheets from both the outgrowth and the surface epithelium, or by collagenase from the remaining stroma, were seeded on 3T3 feeder layers.

RESULTS

Intrastromal invasion was verified in all seven explant cultures by positive pan-cytokeratin staining. Immunofluorescence staining revealed that invaded epithelial cells were positive for p63α and ΔNp63, with or without nuclear staining of Pax6. Double immunostaining to CK10 and CK12 revealed that squamous metaplasia induced by AL was noted on the surface epithelium but not in intrastromally invaded epithelial cells. On 3T3 feeder layers, both the outgrowth and the surface epithelium yielded significant numbers of holoclones and meroclones positive to ΔNp63 but negative to CK10 and CK12. In contrast, intrastromally invaded epithelial cells generated only paraclones negative to ΔNp63 and CK12 but positive to CK10 regardless of culturing conditions.

CONCLUSIONS

Intrastromal invasion by limbal basal epithelial progenitor cells is universal in all explant culture conditions, explaining why there is a gradual decline of outgrowth potential. Alteration of the limbal stromal niche leads invaded epithelial cells to adopt an epidermal fate.

In vitro cell culture models to study the corneal drug absorption

INTRODUCTION

Many diseases of the anterior eye segment are treated using topically applied ophthalmic drugs. For these drugs, the cornea is the main barrier to reaching the interior of the eye. In vitro studies regarding transcorneal drug absorption are commonly performed using excised corneas from experimental animals. Due to several disadvantages and limitations of these animal experiments, establishing corneal cell culture models has been attempted as an alternative.

AREAS COVERED

This review summarizes the development of in vitro models based on corneal cell cultures for permeation studies during the last 20 years, starting with simple epithelial models and moving toward complex organotypical 3D corneal equivalents.

EXPERT OPINION

Current human 3D corneal cell culture models have the potential to replace excised animal corneas in drug absorption studies. However, for widespread use, the contemporary validation of existent systems is required.

Ex vivo expansion of corneal stem cells on amniotic membrane and their outcome

OBJECTIVES

To identify the stem-cell property of the ex vivo expansion of limbal stem cells (LSCs) on amniotic membrane (AM) in culture system and after clinical transplantation.

METHODS

Four key factors have to be performed in the defined culture system: (1) the label-retaining cells have to be identified; (2) the cells can be serially expanded and passaged in vitro; (3) the expanded cells can be labeled by tissue-specific keratin or markers, and (3) their stem cells cannot be labeled by those keratin or markers.

RESULTS

The ex vivo-expanded LSCs on AM were positive for p63 and ABCG2 and BrdU label-retaining studies on flat mount preparation. When the ex vivo-expanded LSCs with AM were transplanted into a subcutaneous layer of nude mice, they formed multiple layers of cells. Only the basal layer of cells was positive for p63 and BrdU. The cells over the suprabasal layers were positive for K12/K3. The pathologic studies of corneal specimen of successful LSC transplantation after penetrating keratoplasty demonstrated that P63-positive cells were noticed all over the basal layer of central cornea and AM could be identified at 10 months after LSC transplantation.

CONCLUSIONS

These results indicate that the AM provided the niche function for cultured LSCs and maintained the limbal-like environment for the transplanted area of cornea. The survival of cases depends on the severity of the disease entity, culture technique, and maintenance of the niche environment for LSCs in the culture and after clinical transplantation.

Effects of fibroblast origin and phenotype on the proliferative potential of limbal epithelial progenitor cells

The current study investigates potential differences in fibroblast phenotype across the anterior segment of the human eye with the aim to understanding factors that support the regenerative function of human limbal epithelial progenitor cells (LEPs) during wound healing. Separate cultures of fibroblasts were established from the cornea, limbus and sclera by growth in serum-supplemented medium. The resulting cultures were examined for potential differences in morphology and growth rate, as well as expression of CD34, CD45, CD90, CD141, CD271, vimentin and α-smooth muscle actin (α-sma). Finally, cultures were examined for their ability to support the growth of LEPs. While all cultures grew at a similar rate, scleral cultures often contained larger and more irregularly shaped cells which stained positive for α-sma. Western blotting confirmed a gradient of α-sma expression with lowest levels in corneal cultures. All three cultures stained positively for CD90 and vimentin, and were negative for CD34, CD45, CD141 and CD271. Only limbal or corneal irradiated fibroblasts supported the establishment of LEP cultures. While LEP colony forming efficiency and prominent expression of ABCG2, C/EBPδ and p63 was similar with either limbal or corneal fibroblasts, limbal fibroblasts supported significantly better growth. These results indicate that scleral fibroblasts have an increased capacity for myofibroblast formation which appears to negatively impact on their ability to support LEP growth. Superior growth of LEPs in the presence of limbal fibroblasts indicates a role for limbal fibroblasts in promoting the proliferation of limbal epithelium during wound healing.

IL6 and the human limbal stem cell niche: a mediator of epithelial-stromal interaction

The corneal epithelium is maintained by the limbal epithelial stem cells (LESCs). In this study, an in vitro model is proposed for the investigation of cell-cell interactions involving LESC maintenance. Imaging of the limbal niche demonstrated close spatial arrangement between basal limbal epithelial cells within putative LESC niche structures and fibroblasts in the stroma. Interactions of the human limbal epithelial (HLE) cells and mitotically active human limbal fibroblasts (HLF) were studied for the first time in a serum-free in vitro model that simulated aspects of the limbal niche microenvironment. HLE cocultured in a ratio 3:1 with HLF exhibited enhanced expression of the putative stem cell markers ABCG2 and p63α and holoclones were preserved as shown by colony-forming efficiency assays, clonal analysis, and colony characterisation. Interleukin 6 (IL6) was found to be up-regulated in the 3.1SF system when compared to the HLE culture with growth-arrested fibroblasts and serum (gold standard system, GS). IL6 caused a time-dependent phosphorylation of STAT3 in HLE cells. STAT3 and IL6 inhibition in 3.1SF cultures significantly reduced HLE colony-forming efficiency, suggesting a previously undetected STAT3-mediated involvement of IL6 in the maintenance of HLE cells in a progenitor-like state.

Importin 13 serves as a potential marker for corneal epithelial progenitor cells

Importin13 (IPO13), the newest member of importin-beta family discovered recently, is a unique nucleus-cytoplasm bidirectional transport receptor protein. In this study, IPO13 expression in human corneal tissue, limbal epithelial primary explant and clonal culture was evaluated by immunostaining and reverse-transcription polymerase chain reasgon. IPO13 function was evaluated in the corneal epithelial culture treated with IPO13 inhibitor, or fetal bovine serum (FBS)-containing Dulbecco's modified Eagle's medium (DMEM) medium by colony-forming efficiency, clone growth capacity, MTT, immunostaining, and Western blotting assay. IPO13 protein was expressed mainly in nuclei of limbal epithelial basal cells, but not in the other cell layers of limbus and full thickness of corneal epithelia. IPO13 was expressed in the majority of epithelial cells in early-stage clones and in the margin of late-stage clones. IPO13 was positively expressed in mouse TKE2 progenitor cells cultured in keratinocyte serum-free defined medium, while it became negative in FBS-containing DMEM, which promoted TKE2 cell differentiation. In the presence of IPO13 inhibitor, IPO13 expression and the proliferative capacity decreased in human limbal epithelial clones and mouse TKE2 cells, which were accompanied with the cell differentiation. In conclusion, our findings demonstrate for the first time that IPO13 is uniquely expressed by human limbal basal epithelial cells, and plays an important role in maintaining the phenotype, high proliferative potential, and less differentiation of corneal epithelial progenitor cells, suggesting that IPO13 could serve as a novel potential marker for corneal epithelial progenitor cells.

Corneal Epithelial Cultures Generated from Organ-Cultured Limbal Tissue: Factors Influencing Epithelial Cell Growth

PURPOSE

To explore the in vitro proliferative potential of human limbal epithelial cells after 31 degrees C organ-culture storage and to investigate putative factors influencing it.

METHODS

185 cultures of limbal explants were carried-out either from full-thickness explants (n = 102) or from enzymatically dissociated cells (n = 83) seeded on a feeder layer of human keratocytes. Epithelial outgrowth was assessed by phase contrast microscopy using a computerized image analysis software. Cell phenotype was evaluated by transmission electron microscopy and immunocytology. Univariate and multivariate analysis were performed to determine factors influencing epithelial growth in culture.

RESULTS

An epithelial outgrowth of 100 square mm or more was observed in 52% of cultures, (average growth area: 440 +/- 256 mm at three weeks). Corneal epithelial phenotype was confirmed by transmission electron microscopy, and cytokeratin pattern. Cytokeratine 19, deltaNp63, nestin and vimentin positive staining revealed undifferentiated epithelial cells in both explant and cell suspension cultures at three weeks. Short death to cornea retrieval time (p < 0.03) and female donors (p < 0.01) were associated with higher cell growth. Enzymatic treatment of explants by trypsin, but not dispase, decreased cell proliferation at two (p < 0.03) and three weeks (p < 0.04). Donor age, duration of corneal storage, and source of the explant did not influence the cell growth.

CONCLUSION

Organ-culture conditions can preserve limbal cell mitotic potential if limbal tissue is excised early after circulatory arrest. Human keratocytes can be used as a feeder layer allowing epithelial cells to maintain poorly differentiated phenotype in culture. Further investigations are needed to explain the influence of the donor sex on epithelial cell growth in culture.

Effects of different extracellular matrices and co-cultures on human limbal stem cell expansion in vitro

To elucidate the effect of extracellular matrices (ECMs) and related and nonrelated-limbal feeder cells as substitutes for the in vivo niche on the phenotype and genotype of the limbal stem cell (SC) expansion in vitro, human limbal SCs were used. The limbus explants were expanded on human amniotic membrane (AM), commercial ECMs including matrigel (MAT), collagen (COL), and control (no ECM) in presence and absence of feeder cells including human limbal fibroblasts (LFs), a limbus-specific cell and mouse embryonic fibroblasts (MEFs). Proliferation, cell death, immunocytochemistry, expression of specific genes, ultrastructural characteristics, and size and granularity of expanded human limbal SCs in different groups were evaluated. The growth, cell proliferation, and survival of limbal SCs were enhanced by AM and MAT matrices. Ultrastructure and expression of stemness markers revealed that there was no significance difference between AM and MAT. However, flow cytometric analysis showed that the size and granularity of cultured cells increased in the presence of MAT and COL as well as in no ECM group. Moreover, co-culturing of limbal explants with LFs and MEFs on AM and MAT groups, enhanced the expansion and survival of cultured cells in comparison with others. In conclusion, the cultivation of human limbal explants on AM co-culturing with human LFs promises to be a good model for preparing undifferentiated epithelial sheets suitable for transplantation.

Stem cells of the adult cornea: from cytometric markers to therapeutic applications

The cornea is a major protective shield of the interior of the eye and represents two thirds of its refractive power. It is made up of three tissue layers that have different developmental origins: the outer, epithelial layer develops from the ectoderm overlying the lens vesicle, whereas the stroma and the endothelium have mesenchymal origin. In the adult organism, the outermost corneal epithelium is the most exposed to environmental damage, and its constant renewal is assured by the epithelial stem cells that reside in the limbus, the circular border of the cornea. Cell turnover in the stromal layer is very slow and the endothelial cells probably do not reproduce in the adult organism. However, recent experimental evidence indicates that stem cells may be found in these layers. Damage to any of the corneal layers leads to loss of transparency and low vision. Corneal limbal stem cell deficiency results in severe ocular surface disease and its treatment by transplantating ex vivo expanded limbal epithelial cells is becoming widely accepted today. Stromal and endothelial stem cells are potential tools of tissue engineering and regenerative therapies of corneal ulcers and endothelial cell loss. In the past few years, intensive research has focused on corneal stem cells aiming to improve the outcomes of the current corneal stem cell transplantation techniques. This review summarizes the current state of knowledge on corneal epithelial, stromal and endothelial stem cells. Special emphasis is placed on the molecular markers that may help to identify these cells, and the recently revealed mechanisms that could maintain their "stemness" or drive their differentiation. The techniques for isolating and culturing/expanding these cells are also described.

A novel NIH/3T3 duplex feeder system to engineer corneal epithelial sheets with enhanced cytokeratin 15-positive progenitor populations

Corneal epithelial cell sheets co-cultivated with feeder cells are used to reconstruct the ocular surface in stem cell-depleted eyes. The present study was conducted to investigate the optimal method of using feeder cells in the interest of preserving progenitor cells in cultivated sheets. We compared the phenotype and secondary colony forming efficiency (CFE) of cell sheets that were engineered using 3T3 feeder cells as a separate layer or as a contact layer. We also devised a novel "duplex feeder" system that consists of two separate layers of feeder cells. After cells reached confluence, cells were cultured at the air-liquid interface to allow full stratification. Stratified sheets were then analyzed using immunohistochemistry and secondary colony formation. Contact feeder cultures and duplex feeder cultures yielded epithelial sheets with small, cuboid basal cells with strong expression of keratin (K)3, K12, and K 15. Furthermore, only duplex feeder layers reproduced the basal K 15, suprabasal K12 limbal phenotype where epithelial stem cells reside. A similar effect was observed when cornea stroma-derived progenitor cells were used as feeder cells. Duplex feeder sheets also produced significantly more secondary colonies than cells dissociated from single layer sheets, suggesting that the duplex feeder system produces transplantable sheets with a higher yield of progenitor cells.

Hyperosmolarity-induced cornification of human corneal epithelial cells is regulated by JNK MAPK

PURPOSE

To evaluate the effects of hyperosmolar stress on expression of cornified envelope (CE) precursors and transglutaminases (TGs) by primary cultured human corneal epithelial (PCHCE) cells and the regulatory effects of JNK MAPK on this process.

METHODS

Expression of CE precursors and TGs were evaluated in PCHCE cells exposed to media of increasing osmolarity (350-450 mOsM) for 24, 48, and 72 hours. JNK1 and -2 MAPKs were inhibited by addition of short interfering (si)RNA. Relative levels of mRNA transcripts and proteins were evaluated. TG activity, cell viability, and apoptosis were detected in PCHCE cells, with or without siRNA-JNKs.

RESULTS

Exposure of PCHCE cells to hyperosmolar medium increased TG activity at 3 hours, levels of the CE precursors SPRR1b and -2a and membrane-associated TG1 mRNA at 6 hours, and tissue-type TG2 mRNA at 24 hours. Osmotic stress decreased corneal epithelial cell viability, which was due in part to stimulation of apoptosis and cornification death. Inhibiting JNK2 production by siRNA in osmotically stressed PCHCE cells prevented the stimulation of SPRR and membrane-associated TG1 production and TG activity, and improved cell viability, whereas inhibition of JNK1 prevented early apoptosis.

CONCLUSIONS

Osmotic stress promotes production of certain CE proteins and cross-linking membrane-associated TG1 and decreases cell viability via JNK MAPK-mediated pathways. Strategies that inhibit JNK production downregulate the cornification response of PCHCE cells to osmotic stress. These findings have potential therapeutic implications for preventing cornification of the corneal epithelium in response to the hyperosmolar tear film in dry eye disease.

Corneal epithelial cell cultures as a tool for research, drug screening and testing

Understanding of visual system function and the development of new therapies for corneal diseases and damages depend upon comprehension of the biological roles of the tissue. The in vitro cultivation of corneal epithelial cells and cell lines derived from them has become a powerful tool to analyze and understand such issues. Currently, researchers have developed well-defined and precisely described culture protocols and a collection of corneal epithelial cell lines. These cell lines have been obtained through different experimental approaches: (1) the ectopic expression of oncogenes, (2) the inactivation of p16 and p53 pathways and hTERT expression, and (3) the spontaneous establishment after serial cultivation of cells. The advantages or disadvantages for these approaches are discussed. In conclusion, the availability of several culture protocols and immortalized cell lines that express corneal epithelial phenotype will be useful for investigating issues such as gene regulation and tissue development, or for validating alternative methods in toxicology.

Human amniotic epithelial cells as novel feeder layers for promoting ex vivo expansion of limbal epithelial progenitor cells

Human amniotic epithelial cells (HAECs) are a unique embryonic cell source that potentially can be used as feeder layers for expanding different types of stem cells. In vivo, HAECs uniformly expressed pan-cytokeratins (pan-CK) and heterogeneously expressed vimentin (Vim). The two phenotypes expressing either pan-CK(+)/Vim(+) or pan-CK(+)/Vim(-) were maintained in serum-free media with high calcium. In contrast, all HAECs became pan-CK(+)/Vim(+) in serum-containing media, which also promoted HAEC proliferation for at least eight passages, especially supplemented with epidermal growth factor and insulin. Mitomycin C-arrested HAEC feeder layers were more effective in promoting clonal growth of human limbal epithelial progenitors than conventional 3T3 murine feeder layers. Cells in HAEC-supported clones were uniformly smaller, sustained more proliferation, and expressed less CK12 and connexin 43 but higher levels of stem cell-associated markers such as p63, Musashi-1, and ATP-binding cassette subfamily G2 than those of 3T3-supported clones. Subculturing of clonally expanded limbal progenitors from HAEC feeder layers, but not from 3T3 feeder layers, gave rise to uniformly p63-positive epithelial progenitor cells as well as nestin-positive neuronal-like progenitors. Collectively, these results indicated that HAECs can be used as a human feeder layer equivalent for more effective ex vivo expansion of adult epithelial stem cells from the human limbus. Disclosure of potential conflicts of interest is found at the end of this article.

The fate of limbal epithelial progenitor cells during explant culture on intact amniotic membrane

PURPOSE

The clinical success of treating corneas with total limbal stem cell deficiency using limbal biopsy explants cultured on intact amniotic membrane (iAM) relies on ex vivo expansion of limbal epithelial progenitor cells. However, the ultimate fate of limbal epithelial progenitor cells in the explant remains unclear.

METHODS

Human limbal explants were cultured on iAM for 2 weeks and then removed and transferred to a new iAM until passage 3. The outgrowth surface area of each passage was measured and compared. For each passage, clonogenicity on 3T3 fibroblasts feeder layers was compared among progenitor cells removed from the outgrowth, the explant surface, and the remaining stroma. Cryosections of the explant and the outgrowth were detected with p63, vimentin, pancytokeratin, and the basement membrane components type VII and IV collagen and laminin 5 antibodies.

RESULTS

The outgrowth surface area significantly decreased from passage (P)1 to P3. The total number of epithelial cells that were isolated from the explant surface also decreased from before culture (P0) to P1, became stable from P1 to P2, but was uncountable at P3. Clonogenicity significantly declined from P1 to P3 for the epithelium derived from the explant surface and the outgrowth epithelium; the extent was less in the former than in the latter at P2 and P3. In addition, groups of epithelial cells invaded the limbal stroma of the explants from P1 to P3; p63(+)/pancytokeratin(-) and p63(+)/vimentin(+) cells also presented in the limbal stroma. Increasing fibroblast, but not epithelial, colonies were observed from cells isolated from the remaining limbal stroma when seeded on 3T3 fibroblast feeder layers from P1 to P3.

CONCLUSIONS

During ex vivo expansion on iAM, some limbal epithelial progenitor cells indeed migrate onto iAM from the explant surface, whereas some also invade the limbal stroma, very likely undergoing epithelial-mesenchymal transition. This new information should be taken into account in formulating new strategies to improve the expansion protocol.

Interaction between stromal fibroblasts and colorectal cancer cells in the expression of vascular endothelial growth factor

BACKGROUND

Vascular endothelial growth factor (VEGF), a potent angiogenic factor, has been implicated in metastasis of colorectal cancer (CRC). The present study aimed to clarify whether cancer-stromal interaction induces the production of VEGF.

MATERIALS AND METHODS

Human colonic fibroblasts (CCD-18Co) and CRC (SW480, SW620) cells were analyzed in this study. The cell cycle of colonic fibroblasts during co-culture was analyzed by flow cytometry. VEGF and TGF-beta1 released into the conditioned media in co-culture models were measured. Northern blot with human specific VEGF probe was performed to identify the expression of VEGF in this model.

RESULTS

Co-culture of colonic fibroblasts with CRC cells increased the viability of fibroblasts during co-culture. Cell cycle analysis revealed that most of the fibroblasts co-cultured with CRC cells were arrested at G1 phase and few cells were in sub-G1 phase that indicates apoptosis. Although VEGF protein was detected in the culture media of all of the monocultures, co-cultivation of CRC with fibroblasts resulted in synergistic increase of VEGF production compared with monocultures. However TGF-beta1 protein was not detected in any conditioned medium. VEGF mRNA was detected in both CRC and fibroblasts. Under co-culture condition, an abundance of VEGF mRNA expression was noted in fibroblasts relative to CRC cells.

CONCLUSIONS

The present study suggests that CRC manipulates the host stroma to suppress apoptosis and up-regulate VEGF production.

Isolation of putative corneal epithelial stem cells from cultured limbal tissue

Purpose

To investigate methods of isolating putative corneal epithelial stem cells from cultured limbal tissue.

Methods

Three extraction techniques were compared to identify an efficient method of obtaining a large number of viable corneal epithelial stem cells from the limbus. Limbal tissues were extracted by incubation at 37℃ or 4℃ for 1 or 16 hours, respectively, with 1.2U/ml dispase/trypsin or by treatment with 0.05% trypsin and 0.01% ethyldiaminetetraacetic acid (EDTA) at 37℃ in single procedure. Collected cells were cultured on NIH/3T3-seeded plates, and colony forming efficiency (CFE) was evaluated. Fluorescence activated cell sorting (FACS) was performed with a Coulter EPICS 753 after incubation with Hoechst 33342 and propidium iodide (PI). Hoechst negative cells were obtained using gates exhibiting low Hoechst blue with a 424/44 nm BP filter. Gated cells of each fraction were re-cultured to assess the capability of colony formation.

Results

The mean numbers of viable cells obtained from treatment with dispase and trypsin was 3×10⁴ cell/ml and 8.06×10⁵ cell/ml at 37℃and 4℃ incubations; the number increased to 1.21×10⁶ cell/ml with a trypsin/EDTA treatment (p<0.05). CFE was 9.67±2.13% and 6.63±2.35% in rabbit and human cells, respectively. Likewise, the Hoechst negative fraction was 3.61±0.42% and 5.21±4.91% in rabbit and human cells, respectively. The sorted Hoechst negative cells were cultured through four passages, forming small round colonies. In rabbit cells, the CFEs of Hoechst negative and positive fractions after FACS, were 12.67±2.24% and 1.17±6.13%, respectively (p<0.05).

Conclusions

Putative corneal epithelial stem cells were efficiently isolated from limbal tissue using a trypsin/EDTA extraction and FACS. This technique may be very useful in tissue engineered stem cell therapy.

Improved transduction of human corneal epithelial progenitor cells with cell-targeting adenoviral vectors

The development of vectors and techniques to transfer therapeutic genes to corneal epithelium has broad clinical applications. To determine if adenoviral (Ad5) vectors could be tailored to increase transduction of corneal epithelial progenitor cells expressing epidermal growth factor receptor (EGFR), the feasibility of targeting gene therapy vectors to genetically modify primary cultured human corneal epithelial cells (PHCEC) was evaluated. PHCECs were cultured from human limbal explants and transduced with Ad5 vectors containing the enhanced green fluorescent protein (GFP) reporter cassette to mediate gene transfer. The efficiencies of transduction with different Ad5 dosages and different time periods of exposure were compared. Metabolically biotinylated Ad5 vectors were retargeted to PHCECs using biotinylated epidermal growth factor (EGF) as a cell-targeting ligand. Phenotypes and function assays of transduced cells were determined by real-time PCR and BrdU incorporation. Ad5 vectors transduced approximately 50-93% of PHCEC at 10-100 PFU/cell in a dose-dependent manner and the transgene persisted for more than 2 weeks in vitro. Retargeting of biotinylated Ad5 with EGF increased transduction of EGFR and ABCG2-expressing corneal epithelial progenitor cells up to nine-fold and reduced transduction of K12 and involucrin-expressing differentiated corneal epithelial cells and had higher BrdU incorporation indexes. These data provide proof of principle that ligand-bearing modified Ad5 vectors can target a population of corneal epithelial progenitor cells for corneal gene therapy.

Gap junction protein connexin 43 serves as a negative marker for a stem cell-containing population of human limbal epithelial cells

This study evaluated whether the gap junction protein connexin (Cx) 43 could serve as a negative cell surface marker for human corneal epithelial stem cells. Cx43 expression was evaluated in corneo-limbal tissue and primary limbal epithelial cultures. Immunofluorescent staining and laser scanning confocal microscopy showed that Cx43 was strongly expressed in the corneal and limbal suprabasal epithelial cells, but the basal cells of the limbal epithelium were negative. Cx43 antibody stained mainly large cells but not small cells in primary limbal epithelial cultures. As determined by semiquantitative reverse transcription polymerase chain reaction (PCR) and real-time PCR, Cx43 mRNA was more abundant in the corneal than limbal epithelia, and it was expressed in higher levels in mature limbal epithelial cultures. Using GAP11, a rabbit polyclonal antibody against the Cx32 extracellular loop 2 (151-187), a sequence that is highly homologous in Cx43, the Cx43(dim) and Cx43(bright) cells were selected from primary limbal epithelial cultures by fluorescence-activated cell sorting and were evaluated for stem cell properties. These Cx43(dim) and Cx43(bright) cells were confirmed by their expression levels of Cx43 protein and mRNA. The Cx43(dim) cells were found to contain higher percentages of slow-cycling bromodeoxyuridine (BrdU)-label retaining cells and the cells that were positive for stem cell-associated markers p63, ABCG2, and integrin beta1 and negative for differentiation markers K3 and involucrin. The Cx43(dim) cells possessed a greater proliferative potential than Cx43(bright) cells and nonfractionated cells as evaluated by BrdU incorporation, colony-forming efficiency, and growth capacity. Our findings suggest that human limbal basal cells do not express connexin 43, which could serve as a negative cell surface marker for the stem cell-containing population of human limbal epithelial cells.

Cell size correlates with phenotype and proliferative capacity in human corneal epithelial cells

This study investigated whether cell size correlates with phenotype and proliferative capacity in human corneal epithelial cells. Primary cultured human corneal epithelial cells were sorted by flow cytometry based on forward scatter profile in comparison with the profile of beads of known size. Four fractions (A, B, C, and D) of cells ranging in size from 10 to 16, 17 to 23, 24 to 30, and >or=31 microm in diameter, respectively, were collected to evaluate their 5-bromo-2-deoxyuridine (BrdU) label retention properties, cell phenotype, and clonal growth capacity on a 3T3 fibroblast feeder layer. Among these four populations, cell size was shown to positively correlate with the expression of the differentiation markers keratin (K) 3, K12, and involucrin and inversely with the levels of stem cell-associated markers DeltaNp63 and ABCG2 and with colony-forming efficiency (CFE) and growth capacity. Population A with the smallest size, accounting for 11.0%+/-4.5% of the entire population, contained the greatest number of BrdU label-retaining slow-cycling cells, displayed the highest percentage of cells immunopositive to p63 and ABCG2 and negative to K3 and involucrin, expressed the highest levels of DeltaNp63 and ABCG2 mRNA and the lowest levels of K3, K12, and involucrin, and possessed the highest CFE and growth capacity. These findings suggest that cell size correlates with cell differentiation phenotypes and proliferative capacity in human corneal epithelial cells. The smallest cells in population A seem to be enriched for putative stem cells, and small cell size may represent one of the important properties of adult corneal epithelial stem cells.