Ocular surface epithelia contain ABCG2-dependent side population cells exhibiting features associated with stem cells

Lycopersicon esculentum lectin is a marker of transient amplifying cells in in vitro cultures of isolated limbal stem cells

The maintenance of a healthy corneal epithelium under both normal and wound healing conditions is achieved by a population of stem cells (SCs) located in the basal epithelium at the corneoscleral limbus. In the light of the development of strategies for reconstruction of the ocular surface in patients with limbal stem cell deficiency, a major challenge in corneal SCs biology remains the ability to identify stem cells in situ and in vitro. To date, not so much markers exist for the identification of different phenotypes. CESCs (corneal epithelial stem cells) isolated from limbal biopsies were maintained in primary culture for 14 days and stained with Hoechst and a panel of FITC-conjugated lectins. All lectins, with the exception of Lycopersicon esculentum, labelled CESCs irrespective of the degree of differentiation. Lycopersicon esculentum, that binds N-acetylglucosamine oligomers, labelled intensely only the surface of TACs (single corneal epithelial stem cells better than colonial cells). These results suggest that Lycopersicon esculentum lectin is a useful and easy-to-use marker for the in vitro identification of TACs (transient amplifying cells) in cultures of isolated CESCs.

Ex vivo cultured limbal epithelial transplantation. A clinical perspective

The term ex vivo cultured limbal epithelial transplantation (CLET) refers to the process of culturing a sheet of human limbal epithelium in the laboratory and transplanting this sheet back onto the limbal stem cell-deficient cornea of the same patient or another recipient. This emerging technology represents one of the earliest successes in regenerative medicine. CLET is, at present, best suited to patients who have unilateral total limbal stem cell deficiency arising from chemical injury and who are suitable for autologous cell culture and transplantation. Although the results of allogeneic cell transplantation are encouraging and superior to conventional stem cell transplantation techniques, insufficient follow-up precludes conclusions regarding the long-term outcomes. Other tissues, such as oral mucosal epithelium, are emerging as viable alternative sources of cells, especially for patients with bilateral disease.

Molecular signatures and biological pathway profiles of human corneal epithelial progenitor cells

Identification and isolation of adult stem cells are still challenging for stem cell biologists. For example, no consensus exists yet regarding definitive markers for corneal epithelial stem cells, which have been identified to reside in the limbus for two decades. This study characterized the molecular signatures and biological pathways of limbal epithelial progenitors, the rapid adherent cells (RAC) isolated by adhesion on collagen IV, using human genome microarrays, real-time PCR and immunofluorescent staining. The microarrays produced highly reproducible data not only for all gene transcripts, but also for significantly changed genes, although the total 12 samples of 3 cell populations in 2 arrays were isolated from 4 separate experiments at different time period. The hierarchical clustering heatmap visually revealed that RAC progenitor population displayed distinguishably characteristic gene expression profile. With verification of 27 important genes by quantitative real-time PCR, the microarray data not only confirm the expression patterns of 15 known genes as stem cell associated markers representing limbal stem cell phenotype, but also identified many significantly regulated genes expressed by limbal progenitor cells. Transcription factor TCF4 and cell surface protein SPRRs were identified as potentially positive or negative markers, respectively, for corneal epithelial progenitor cells. Using GenMAPP and MAPPFinder, we have identified three patterns of biological pathway profiles, overexpressed, underexpressed and balanced, by RAC progenitors based on gene ontology categories. These genes and related pathways are interesting targets for further identification and isolation of limbal stem cells as well as other tissue-specific adult stem cells.

Effluxing ABC transporters in human corneal epithelium

ATP-binding cassette (ABC) transporters are able to efflux their substrate drugs from the cells. We compared expression of efflux proteins in normal human corneal epithelial tissue, primary human corneal epithelial cells (HCEpiC), and corneal epithelial cell culture model (HCE model) based on human immortal cell line. Expression of multidrug resistance protein 1 (MDR1), multidrug resistance-associated protein 1-6 (MRP1-6) and breast cancer resistance protein (BCRP) was studied using quantitative RT-PCR, Western blot, and immunohistochemistry. Only MRP1, MRP5, and BCRP were expressed in the freshly excised human corneal epithelial tissue. Expression of MRP1 and MRP5 was localized predominantly in the basal cells of the central cornea and limbus. Functional efflux activity was shown in the cell models, but they showed over-expression of most efflux transporters compared to that of normal corneal epithelium. In conclusion, MRP1, MRP5, and BCRP are expressed in the corneal epithelium, but MDR1, MRP2, MRP3, MRP4, and MRP6 are not significantly expressed. HCE cell model and commercially available primary cells deviate from this expression profile.

ABCG2: a potential marker of stem cells and novel target in stem cell and cancer therapy

ABCG2 is a member of the ATP binding cassette (ABC) transporters, which can pump a wide variety of endogenous and exogenous compounds out of cells. Widely expressed in stem cells, ABCG2 is also found to confer the side population phenotype and is recognized as a universal marker of stem cells. Although the precise physiological role of ABCG2 in stem cells is still unclear, existing data strongly suggest that ABCG2 plays an important role in promoting stem cell proliferation and the maintenance of the stem cell phenotype. In addition, ABCG2 is also found to be expressed in a number of cancer cells and appears to be a marker of cancer stem cells. Moreover, ABCG2 expression in tumors may contribute to their formation and progression. Thus, ABCG2 has potential applications in stem cell and tumor therapy.

Tissue engineering for conjunctival reconstruction: established methods and future outlooks

Reconstruction of the conjunctiva is an essential part of ocular surface regeneration, especially if an extensive area or the whole ocular surface is affected, such as in patients with ocular cicatricial pemphigoid, Stevens-Johnson syndrome, toxic epidermal necrolysis, or chemical/thermal burns. In these situations, corneal reconstruction almost inevitably fails unless the conjunctival surface is first repaired and a deep fornix is restored. The growing field of tissue engineering and advances in stem cell research offer promising new alternatives for these challenges. This article reviews the present approaches for reconstruction of the conjunctival surface, considering the established strategies and new potential methodologies.

Conjunctival epithelial cells maintain stem cell properties after long-term culture and cryopreservation

Aim: Transplantation of tissue-engineered conjunctival epithelial cell sheets has proven to be a promising technique for conjunctival reconstruction. The ability to cryopreserve conjunctival epithelial cells and maintain their stem cell population would improve their availability for clinical use. The aim of this study was to evaluate whether cryopreservation and long-term in vitro culture has an effect on the proliferative capacity and the progenitor-like cell characteristics of conjunctival epithelial cells. Method: Human conjunctival cells from bulbar biopsies were isolated and expanded on a growth arrested 3T3 feeder layer. The cells were evaluated for cytokeratin (CK4/CK19) expression by immunostaining. An aliquot with half of the cells from the initial culture was frozen in liquid nitrogen and stored for 14 days and, in addition, donor cells were cryopreserved for more than 6 months (202.7 ± 13.0 days). Both cryopreserved and noncryopreserved cells were serially cultivated over four passages. For each passage the colony-forming efficiency and the cell population doubling rates were evaluated, and expression of putative progenitor cell markers, p63α and ABCG2, was assessed by immunostaining and reverse transcription PCR. Results: Both noncryopreserved and cryopreserved cells demonstrated a high colony-forming capacity that decreased with passage. Cells from both groups underwent approximately 20 cell population doublings before senescence. Immunoreactivity to p63α and ABCG2 was found in both groups until passage 4 and their presence was also confirmed by reverse transcription PCR. No difference in cell viability, colony-forming efficiency and immunoreactivity to p63α and ABCG2 was observed between cells cryopreserved for 14 days, and more than 6 months (202.7 ± 13.0 days). Conclusion: Conjunctival epithelial cells with progenitor cell-like characteristics can be efficiently cryopreserved and can subsequently maintain their function in vitro over several culture passages. The option to cryopreserve conjunctival cells prior to in vitro expansion would be an advantage when cells have to be cultivated for clinical transplantation.

Differential gene expression in the pig limbal side population: implications for stem cell cycling, replication, and survival

PURPOSE

To define the molecular signature of limbal SP cells and identify signaling pathways associated with the phenotype of these putative stem cells.

METHODS

Primary cultures of pig limbal epithelial cells stained with Hoechst 33342 were sorted by flow cytometry into SP and non-SP cells, and purified RNA was processed for microarray analysis with an oligonucleotide spotted array. Expressed transcripts for which SP and non-SP expressions differed by more that 1.5-fold in each paired set and by twofold overall were considered to be differentially expressed. Differential expression was validated by quantitative PCR and immunostaining. Data-mining methods were used to identify cellular processes that are either salient or depressed in the SP cells.

RESULTS

The microarray identified approximately 9000 distinct, expressed, and identifiable genes. Of those, 382 and 296 were either over- or underexpressed in the SP cells, respectively. Overrepresentation analysis indicated that SP cells are in a low metabolic and biosynthetic state. In addition, a pattern of elevated MXD1, MAXI2, DUSP5, p27/KIP1, and p57/KIP2 and decreased Cyclin D and CDK genes can be expected to slow intrinsic and mitogen-induced G(1)-to-S cell cycle transition. SP cells were also rich in genes associated with stem cell phenotype and genes providing protection against oxidative and/or xenobiotic damage.

CONCLUSIONS

Microarray analysis of pig limbal SP cells yielded a molecular signature underscoring a phenotype characterized by slow cycling and low metabolic activity. The results provide valuable insights for the preservation and/or replication of epithelial stem cells.

A contact lens-based technique for expansion and transplantation of autologous epithelial progenitors for ocular surface reconstruction

BACKGROUND

A healthy cornea is reliant on a distinct population of stem cells (SC) that replace damaged or aging epithelium throughout life. Depletion of the SC pool or damage to the niche can result in a blinding and painful condition known as limbal-SC deficiency (LSCD). Although current treatment strategies for reconstituting the ocular surface for patients suffering LSCD are promising, they are complicated by transferring autologous or allogeneic progenitors in the presence of animal, human, and synthetic products. We report on the safe and efficacy of a unique autologous SC transfer technique that utilizes an Food and Drug Administration-approved contact lens (CL) as the SC substrate and carrier for patients with LSCD.

METHODS

Three patients with LSCD due to aniridia (n=1) and posttreatment for recurrent ocular surface melanoma (n=2) were included. Limbal (n=2) or conjunctival biopsies (n=1) were harvested and progenitors expanded ex vivo on therapeutic CLs in the presence of autologous serum. Cell-laden CLs were transferred to the patient's corneal surface and clinical outcome measures were recorded (follow-up range, 8-13 months).

RESULTS

A stable transparent corneal epithelium was restored in each patient. There was no recurrence of conjunctivalization or corneal vascularization, and a significant improvement in symptom score occurred in all patients. Best-corrected visual acuity was increased in all eyes after the procedure.

CONCLUSION

Ex vivo expansion of ocular surface epithelium in the presence of autologous serum and transplantation with the aid of a soft CLs is a promising new technique capable of achieving ocular surface rehabilitation.

Current Prospects for Adult Stem Cell–Based Therapies in Ocular Repair and Regeneration

Recent advances in stem cell biology have led to the exploration of stem cell-based therapies to treat a wide range of human diseases. In the ophthalmic field, much hope has been placed on the potential use of these cells to restore sight, particularly in those conditions in which other established treatments have failed and in which visual function has been irreversibly damaged by disease or injury. At present, there are many limitations for the immediate use of embryonic stem cells to treat ocular disease, and as more evidence emerges that adult stem cells are present in the adult human eye, it is clear that these cells may have advantages to develop into feasible therapeutic treatments without the problems associated with embryonic research and immune rejection. Here we discuss the current prospects for the application of various adult ocular stem cells to human therapies for restoration of vision.

Inhibition of p38 MAPK facilitates ex vivo expansion of skin epithelial progenitor cells

Ex vivo expansion of skin epithelial stem cells has long attracted great interest because of the potential utilization in transplantation and gene therapy. The use of cultured stem or progenitor cells was limited by the lack of applicable culturing system with both satisfactory expansion efficacy and well suppressed differentiation ex vivo. The p38 mitogen-activated protein kinase (MAPK) pathways are responsible for cell growth and differentiation process. We investigated the function of p38 inhibitor SB203580 in the ex vivo expansion of skin epithelial progenitor cells by comparing media with or without addition of this inhibitor. Our results showed that the culturing medium with murine 3T3 feeder layers added with 10 microM SB203580 was more effective in promoting clonal growth of human skin epithelial progenitors or stem cells than the conventional medium without SB203580. The clone initial day in cells treated with 10 microM SB203580 came 2 d earlier with higher colony formation efficiency. The skin epithelial progenitor cells treated with 10 microM SB203580 formed clones that were uniformly smaller in size, longer in sustained proliferation, shorter in clone doubling time, higher in S-phase cells percentage, and lower in levels of differentiation markers such as K10 along with higher levels of stem-cell-associated markers such as p63, K15, and ABCG2 than those cultured in the conventional medium. Collectively, these results indicate that the p38 MAPK pathways inhibitor SB203580 can be used as a culture medium additive that helps to achieve more effective ex vivo expansion of skin epithelial progenitor cells.

Molecular profiling of conjunctival epithelial side-population stem cells: atypical cell surface markers and sources of a slow-cycling phenotype

PURPOSE

Side-population (SP) cells isolated from limbal and conjunctival epithelia derive from cells that are slow cycling in vivo, a known feature of tissue stem cells. The purpose of this study was to define the molecular signature of the conjunctival SP cells and identify markers and signaling pathways associated with the phenotype of these cells.

METHODS

Overnight cultures of freshly isolated human conjunctival epithelial cells stained with Hoechst 33342 were sorted by flow cytometry into SP and non-SP cohorts. Isolated RNA was processed for microarray analysis using a commercial oligonucleotide spotted array. Results were validated at the gene and protein levels by quantitative PCR and immunologic methods. Data mining methods were used to identify cellular processes relevant for stem cell function.

RESULTS

Comparative analyses of transcripts expression based on present and absent software calls across four replicate experiments identified 16,993 conjunctival epithelial transcripts including 10,266 unique known genes of approximately 24,000 represented in the array. Of those genes, 1254 and 363 were overexpressed (>2-fold) or underexpressed (<0.5-fold), respectively, in the SP. The overexpressed set included genes coding for proteins that have been associated with (1) embryonic development and/or stem cell self renewal (MSX, MEIS, ID, Hes1, and SIX homeodomain genes); (2) cell survival (e.g., CYP1A1 to degrade aromatic genotoxic compounds); (3) cycling rate (e.g., DUSPs and Pax6 to foster slow cycling); and (4) genes whose expression is not typical in epithelia (e.g., CD62E).

CONCLUSIONS

The molecular signature of conjunctival SP cells is consistent with a stem cell phenotype. Their gene expression patterns underpin slow cycling and plasticity, features associated with tissue stem cells. The results provide valuable insights for the preservation and/or expansion of epithelial stem cells.

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.

Characterization of putative stem cells in isolated human colonic crypt epithelial cells and their interactions with myofibroblasts

Colonic epithelial stem cells are believed to be located at the crypt base where they have previously been shown to express musashi-1. The colonic stem cell niche, which includes extracellular matrix and myofibroblasts (together with other cell types), is likely to be important in maintaining the function of the progenitor cells. The aims of our studies were to characterize stem cells in isolated and disaggregated human colonic crypt epithelial cells and investigate their interactions with monolayers of primary human colonic myofibroblasts. In unfractionated preparations of disaggregated colonic crypts, musashi-1 positive cells preferentially adhered to colonic myofibroblasts, despite the presence of excess blocking anti-beta(1)-integrin antibody. These adherent epithelial cells remained viable for a number of days and developed slender processes. Cells with side population characteristics (as demonstrated by ability to expel the dye Hoechst 33342) were consistently seen in the isolated colonic crypt epithelial cells. These side population cells expressed musashi-1, beta(1)-integrin, BerEP4, and CD133. Sorted side population crypt epithelial cells also rapidly adhered to primary colonic myofibroblasts. In conclusion, in preparation of isolated and disaggregated human colonic crypts, cells with stem cell characteristics preferentially adhere to primary human colonic myofibroblasts in a beta(1)-integrin-independent fashion.

The effect of amniotic membrane preparation method on its ability to serve as a substrate for the ex-vivo expansion of limbal epithelial cells

Human amniotic membrane (HAM) is employed as a substrate for the ex-vivo expansion of limbal epithelial cells (LECs) used to treat corneal epithelial stem cell deficiency in humans. The optimal method of HAM preparation for this purpose is unknown. This study evaluated the ability of different preparations of stored HAM to serve as substrates for LEC expansion ex-vivo. The effect of removing the amniotic epithelial cells (decellularisation) from HAM prior to seeding of LECs, the effect of glycerol cryopreservation and the effect of peracetic acid (PAA) sterilization and antibiotic disinfection were evaluated using different HAM test groups. Human LECs were cultured on each preparation and the following outcomes were assessed: confluence of growth, cell density, cell morphology and expression of the putative LESC markers deltaN-p63alpha and ABCG2. Removing amniotic epithelial cells prior to seeding of LECs resulted in a higher percentage of confluence but a lower cell density than intact HAM suggesting that decellularisation does not increase proliferation, but rather that it facilitates migration of LECs resulting in larger cells. Decellularisation did not affect the percentage of cells expressing the putative LESC markers deltaN-p63alpha (< or =4% in both intact and acellular groups) and ABCG2 (< or =3% in both intact and acellular groups). Glycerol cryopreservation of HAM resulted in poor morphology and a low proportion of cells expressing deltaN-p63alpha (< or =6%) and ABCG2 (< or =8%). HAM frozen at -80 degrees C in Hank's Balanced Salt Solution (HBSS) was superior, demonstrating excellent morphology of cultured LECs and high levels of deltaN-p63alpha (< or =68%) and ABCG2 (< or =62%) expression (p<0.001). The use of PAA or antibiotics to decontaminate HAM does not appear to affect this function. The variables affecting the ability of HAM to serve as a substrate for LEC expansion ex-vivo are poorly understood. The use of glycerol as a cryoprotectant impairs this ability whereas simple frozen HAM appears to work extremely well for this purpose.

Preferential gene expression in the limbus of the vervet monkey

PURPOSE

To elucidate the unique molecular factors and biological processes that are differentially expressed in the limbal stem cell microenvironment by comparing directly to that of its immediate adjacent structures, the cornea and conjunctiva.

METHODS

Total RNA was isolated and amplified from the limbus, cornea, and conjunctiva. A gene expression profile of each tissue type was obtained by using microarray technique. The transcripts in which the expression level was at least twofold higher than that in the other two tissue types were identified. The expression levels of selected genes were confirmed by quantitative reverse transcription polymerase chain reaction (QRT-PCR). Protein expression of selected genes were confirmed by an immunohistochemistry study in normal human ocular tissue.

RESULTS

There were 186 preferentially overexpressed transcripts in the limbus in direct comparison to that in the cornea and conjunctiva. Many signature genes in the cornea and conjunctiva were among the preferentially expressed transcripts obtained by the microarray data. In addition, a significant number of new genes were identified, and the expression level of all nine selected genes was verified by QRT-PCR. Protein expression levels of keratin 13, tenascin c, homeodomain only protein (HOP), and TP53 apoptosis effector (PERP) were confirmed in human ocular tissues. Functional analysis of the preferentially expressed genes in the limbus reviewed that melanin metabolism and cell-cell adhesion were among the noticeable biological processes. Chromosomal distribution of the overexpressed genes in the limbus was disproportional to that of all known human genes.

CONCLUSIONS

These findings may shed light on the unique molecular components and regulation of limbal stem cells and their niche.

Efficient cultivation conditions for human limbal epithelial cells

To compare the stem niche in different culture conditions of limbal epithelial cells, the suspended human limbal epithelial cells (HLECs) were seeded on the 3T3-pretreated plates and the other suspended cells were plated on amniotic membranes (AMs) which were either cryo-preserved or freeze-dried. All were cultured for 10 to 12 days. Reverse transcription-polymerase chain reaction (RT-PCR) for ATP-binding cassette, subfamily G, member 2 (ABCG2), p63, cytokeratin 12, and connexin 43 were performed in cultivated HLECs and their expression levels were compared. The mRNA expression of all markers examined showed no statistically significant differences between the cells on cryo-preserved and on freeze-dried AM. The expression of p63 and cytokeratin 12 in cultivated cells on AMs were significantly lower than those in 3T3-cocultured cells on RT-PCR and immunofluorescent staining. Cultivated HLECs on AMs showed reduced proliferation and differentiation while maintaining stem-property regardless of the preservative method of AM.

Stem cells in colon cancer

The concept that cancer might arise from a rare population of cells with stem cell-like properties was proposed 150 years ago. Increasing evidence during the past 2 decades suggests the existence of a small subgroup of cells in cancer that are responsible for tumor growth and proliferation. Stem cells have self-renewing properties; thus, they are appealing candidates for generating the malignant phenotype. Although the concept of stem cells in leukemia has received significant attention for more than the past decade, over the past several years, expression of several surface markers on cancer cells has led to identification of tumor-initiating cells in several solid tumors, including melanoma, brain, breast, prostate, liver, pancreatic, ovarian, and recently, colon cancer. This review will provide an update of the biologic basis of the stem cell model and possible targets for the treatment of colon cancer.

Recent progress on tissue-resident adult stem cell biology and their therapeutic implications

Recent progress in the field of the stem cell research has given new hopes to treat and even cure diverse degenerative disorders and incurable diseases in human. Particularly, the identification of a rare population of adult stem cells in the most tissues/organs in human has emerged as an attractive source of multipotent stem/progenitor cells for cell replacement-based therapies and tissue engineering in regenerative medicine. The tissue-resident adult stem/progenitor cells offer the possibility to stimulate their in vivo differentiation or to use their ex vivo expanded progenies for cell replacement-based therapies with multiple applications in human. Among the human diseases that could be treated by the stem cell-based therapies, there are hematopoietic and immune disorders, multiple degenerative disorders, such as Parkinson's and Alzheimer's diseases, type 1 or 2 diabetes mellitus as well as eye, liver, lung, skin and cardiovascular disorders and aggressive and metastatic cancers. In addition, the genetically-modified adult stem/progenitor cells could also be used as delivery system for expressing the therapeutic molecules in specific damaged areas of different tissues. Recent advances in cancer stem/progenitor cell research also offer the possibility to targeting these undifferentiated and malignant cells that provide critical functions in cancer initiation and progression and disease relapse for treating the patients diagnosed with the advanced and metastatic cancers which remain incurable in the clinics with the current therapies.

Characterization of side population cells from human airway epithelium

The airway epithelium is the first line of contact with the inhaled external environment and is continuously exposed to and injured by pollutants, allergens, and viruses. However, little is known about epithelial repair and in particular the identity and role of tissue resident stem/progenitor cells that may contribute to epithelial regeneration. The aims of the present study were to identify, isolate, and characterize side population (SP) cells in human tracheobronchial epithelium. Epithelial cells were obtained from seven nontransplantable healthy lungs and four asthmatic lungs by pronase digestion. SP cells were identified by verapamil-sensitive efflux of the DNA-binding dye Hoechst 33342. Using flow cytometry, CD45(-) SP, CD45(+) SP, and non-SP cells were isolated and sorted. CD45(-) SP cells made up 0.12% +/- 0.01% of the total epithelial cell population in normal airway but 4.1% +/- 0.06% of the epithelium in asthmatic airways. All CD45(-) SP cells showed positive staining for epithelial-specific markers cytokeratin-5, E-cadherin, ZO-1, and p63. CD45(-) SP cells exhibited stable telomere length and increased colony-forming and proliferative potential, undergoing population expansion for at least 16 consecutive passages. In contrast with non-SP cells, fewer than 100 CD45(-) SP cells were able to generate a multilayered and differentiated epithelium in air-liquid interface culture. SP cells are present in human tracheobronchial epithelium, exhibit both short- and long-term proliferative potential, and are capable of generation of differentiated epithelium in vitro. The number of SP cells is significantly greater in asthmatic airways, providing evidence of dysregulated resident SP cells in the asthmatic epithelium. Disclosure of potential conflicts of interest is found at the end of this article.

A method to isolate human limbal basal cells enriched for a subset of epithelial cells with a large nucleus/cytoplasm ratio expressing high levels of p63

The objectives were to develop method of isolating viable human limbal basal cells in order to enrich a subset of small cells with a large Nucleus/Cytoplasm (N/C) ratio expressing high levels of p63, nuclear protein. Limbal tissues were treated with trypsin for 50 min at 37 degrees C in an orbital shaker at 100 rpm with epithelial side down followed by additional 5 min with epithelial side up and then with Dispase II to obtain various epithelial fractions. Isolated cell fractions were assessed for colony forming efficiency and DeltaNp63alpha, connexin (Cx43) mRNA levels. Cytospin smears were double-immunostained for p63 and any one of the stem cell (SC) related markers and analyzed using a laser scanning confocal microscope and advanced image analysis software (Leica Confocal software, 2.61 build 1537 version) for quantification of fluorescence intensity. The isolated limbal basal cells were highly positive for DeltaNp63alpha mRNA but expressing low Cx43 mRNA. They gave rise to higher number of large colonies with compact morphology in contrast to the limbal suprabasal/superficial (LS/S) colonies. Furthermore, a subset with a large N/C ratio expressing high levels of p63 was observed, as much as 25% among the limbal basal cell fraction, in contrast to only about 4% in the total limbal epithelial cells. Such cells were positive for K5 and negative for Ki67, Cx43, and 14-3-3s and were absent in the LS/S fraction. These results collectively substantiate our method of isolation of limbal basal layer cells containing an enriched population of cells with SC phenotype.

Down-regulation of Notch signaling during corneal epithelial proliferation

PURPOSE

We evaluated the expression and activation of Notch pathway genes in the adult human and murine corneal epithelium during proliferation.

METHODS

The expression of Notch pathway genes in the limbal and central human corneal epithelium was compared by reverse transcription polymerase chain reaction (RT-PCR). Their expression pattern was examined by immunofluorescence and in situ hybridization. The temporal expression of Notch1 during murine wound healing was assessed by RT-PCR. Notch activity was determined using western blot for the Notch intracellular domain (NotchIC). The expression of Hes1 was evaluated in cell culture.

RESULTS

The expression of Notch1 and Jagged1 was higher in the human limbal epithelium while the expression of Hes1 and Hes5 was higher in the central cornea. Expression of Notch1, Jagged1, and Hes1 was found predominantly in the basal and immediate suprabasal cells. During neonatal corneal development, NotchIC was detected in occasional cells at P10 while at P15 and P90, it was found in the basal and early suprabasal layers. NotchIC was found to be lower in the limbal compared to central corneal epithelium. The expression of Notch1 was lower at 24 h post-wounding but was completely restored in six days. The levels of NotchIC were decreased at 24 h post-wounding and after application of topical phorbol myristate. In vitro, the expression of Hes1 was higher in confluent cells maintained under high calcium conditions.

CONCLUSIONS

The inverse correlation between Notch signaling and the proliferative status of the corneal epithelium is consistent with the idea that Notch plays a role in corneal epithelial differentiation.

Immunohistochemical markers for corneal stem cells in the early developing human eye

The corneal epithelium is continuously being renewed. Differentiated epithelial cells originate from limbal stem cells (LSCs) located in the periphery of the cornea, the corneoscleral limbus. We have recently identified superoxide dismutase 2 (SOD2) and cytokeratin (CK) 15 as limbal basal cell markers and potential markers for LSCs and early transient amplifying cells in human adults. In this study, we describe the development of the ectodermally derived LSCs and the mesodermally derived niche cells from the time at which the cornea is defined (week 6) until the formation of the early limbal niche (week 14) in human embryos and fetuses. The expression of SOD2 and CK15 was investigated together with other recently identified limbal proteins. Previously suggested LSC and differentiation markers (PAX6, aquaporin-1 and nestin) were also investigated. Both SOD2 and CK15 were present in the corneal epithelium from week 6. However, in week 14 they were predominantly expressed in the limbal epithelium. Both proteins were expressed already from week 7 in a stromal triangular region from which the early mesodermal limbal niche most likely originates. PAX6 was expressed in both ectodermally and mesodermally derived parts of the limbal niche, underscoring the importance of PAX6 in niche formation.

Differences in the protein expression in limbal versus central human corneal epithelium--a search for stem cell markers

In the search for potential limbal stem cell protein markers, the purpose of this study was to characterize differences in protein expression between human central and limbal corneal epithelium by a proteomic approach using two-dimensional polyacrylamide gel electrophoresis (2D PAGE) combined with mass spectrometry (LC-MS/MS). The results were subsequently confirmed by Western blotting and immunohistochemistry. We detected more than 1000 protein spots in each gel. Thirty-two spots were significantly over-expressed in the central part and 70 spots were significantly over-expressed in the limbal part. We identified 25 different proteins. Among these 11 proteins representing different cellular locations and functions were selected for further investigations. Most interestingly, superoxide dismutase 2 (SOD2), was expressed in clusters of cells in the basal limbal epithelium. Heat shock protein 70 protein 1 (HSP70.1) and annexin I were highly abundant in limbal epithelium, although they were also present in the central epithelium to a minor extent. Among the proteins primarily expressed in the limbal fraction we further identified cytokeratin (CK) 15, CK19 and alpha enolase, which have been reported previously to be related to the limbal basal epithelium. The basal limbal epithelium consists of clusters of slow cycling limbal stem cells and rapid cycling transient amplifying cells. Ideally, proteins exclusively expressed in the limbal part of the epithelium may serve as markers for the basal limbal cells. SOD2 and CK15 identify clusters of limbal basal cells and therefore they may serve as markers for limbal stem cells in conjunction with the earliest transient amplifying cells.

Side populations of gastrointestinal cancers are not enriched in stem cells

The side population (SP) phenotype, defined as the reserpine-blockable ability to efflux the nucleic acid dye Hoechst 33342, has been claimed to be enriched for stem cells in several human normal tissues, cancers and cell lines, and thus may be useful for the identification and isolation of cancer stem cells. We demonstrated the presence of SP fractions in all of seven tested gastrointestinal cancer cell lines. Four cell lines were selected (HT29, HGT101, Caco2 and HRA19a1.1) for detailed phenotypic and behavioural analysis with respect to stem cell characteristics. Cell surface marker analysis showed that, contrary to non-SP cells, the SPs entirely lack the expression of CD34. This difference, however, disappeared when the cells were cultured, rendering both populations CD34-positive. Expression of other putative stem cell markers (CD133, CD44, Hes-1, beta-catenin, Musashi-1, Oct-4 and CD117) was identical on SP and non-SPs before and after culturing. Sorted SP and non-SP cells were similarly clonogenic in vitro, tumourigenic in vivo, and displayed similar multipotential differentiation potential in vitro and in vivo. Additionally, culturing cytometrically-sorted SP and non-SP cells showed that the populations are interconvertible, each giving rise to the other. Expression of ABCG2 and Mdr-1, two membrane transporter proteins that have been suggested to be responsible for the drug-effluxing capacities of SP cells, including Hoechst 33342, was identical in non-SP and SP cells, indicating that there may be additional factors responsible for the Hoechst effluxing property in gastrointestinal cancer SP cells. Here, we show that the SP and non-SP fractions, albeit phenotypically distinct populations, do not differ with respect to stem cell-like cell number or behaviour. We thus conclude that the concept of the SP phenotype as a universal marker for stem cells does not apply to gastrointestinal cancer cells. These findings stand in contrast to the observations made in many other tissues and harbour important implications for the future search for intestinal cancer stem cell markers.

Loss of corneal epithelial stem cell properties in outgrowths from human limbal explants cultured on intact amniotic membrane

Background: The corneal epithelium is renewed by stem cells located at the limbus, the so-called limbal stem cells (LSCs). Absence, damage or loss of the LSC population leads to the painful and blinding condition of LSC deficiency (LSCD). Ex vivo expansion of LSCs is an increasingly well recognized treatment modality for LSCD. One method of ex vivo expansion of LSCs involves the culture of limbal explants on amniotic membrane (AM). The purpose of this study was to analyze the outgrowths from human cadaveric limbal explants cultured on AM for properties associated with LSCs. In particular, the expression of putative stem cell markers and the colony-forming efficiency of the different zones of the outgrowth were studied. Methods: The limbal explants were expanded in the standard way used for clinical transplantation and the outgrowths were divided into three zones depending on proximity to the explant (inner, middle and outer zones). The colony-forming efficiencies (CFEs) of cells from each zone were calculated. In addition, the expression of ΔNp63, ABCG2 (both putative positive LSC markers) and cytokeratin K3 (marker of corneal differentiation) were assessed using quantitative reverse transcription PCR (RT-PCR). Immunohistochemistry on paraffin-embedded sections was also performed to demonstrate protein localization and allow further confirmation of the quantitative RT-PCR results. Results: Successful cultures for both the explant outgrowths and the CFE calculations were obtained in every case. CFE showed a successive decline in zones further away from the explant (p < 0.00005). Quantitative RT-PCR revealed that the expression of the positive putative LSC markers ΔNp63 and ABCG2 also showed a steady decrease in the zones furthest from the explant (p < 0.05 and p < 0.005, respectively). The expression of cytokeratin K3 was increased in zones furthest from the explant (p < 0.005). Immunohistochemistry on paraffin-embedded sections of intact ex vivo-expanded limbal epithelium for the putative positive marker p63 and cytokeratin K3 confirmed the findings of the quantitative RT-PCR and CFE results. Conclusions: We demonstrate for the first time that outgrowths from human limbal explants, a widely used technique in ex vivo expansion of LSCs for clinical transplantation, show a steady decline in a wide range of stem cell properties with distance from the central explant. These findings support the importance of proximity of stem cells to their niche environment in maintaining their undifferentiated state. These findings suggest the need for modifications of existing techniques to ensure maximum numbers of LSCs following ex vivo expansion protocols, which will then ensure the success of subsequent engraftment.

Cofilin is a marker of myofibroblast differentiation in cells from porcine aortic cardiac valves

The formation of myofibroblasts in valve interstitial cell (VIC) populations contributes to fibrotic valvular disease. We examined myofibroblast differentiation in VICs from porcine aortic valves. In normal valves, cells immunostained for α-smooth muscle actin (α-SMA, a myofibroblast marker) were rare (0.69 ± 0.48%), but in sclerotic valves of animals fed an atherogenic diet, myofibroblasts were spatially clustered and abundant (31.2 ± 6.3%). In cultured VIC populations from normal valves, SMA-positive myofibroblasts were also spatially clustered, abundant (21% positive cells after 1 passage), and stained for collagen type I and vimentin but not desmin. For an analysis of stem cells, two-color flow cytometry of isolated cells stained with Hoechst 33342 demonstrated that 0.5% of VICs were side population cells; none stained for SMA. Upon culture, sorted side population cells generated ∼85% SMA-positive cells, indicating that some myofibroblasts originate from a rare population with stem cell characteristics. Plating cells on rigid collagen substrates enabled the formation of myofibroblasts after 5 days in culture, which was completely blocked by culture of cells on compliant collagen substrates. Exogenous tensile force also significantly increased SMA expression in VICs. Isotope-coded affinity tags and mass spectrometry were used to identify differentially expressed proteins in myofibroblast differentiation of VICs. Of the nine proteins that were identified, cofilin expression and phospho-cofilin were strongly increased by conditions favoring myofibroblast differentiation. Knockdown of cofilin with small-interfering RNA inhibited collagen gel contraction and reduced myofibroblast differentiation as assessed by the SMA incorporation into stress fibers. When compared with normal valves, diseased valves showed strong immunostaining for cofilin that colocalized with SMA in clustered cells. We conclude that in VICs, cofilin is a marker for myofibroblasts in vivo and in vitro that arise from a rare population of stem cells and require a rigid matrix for formation.

Air exposure induced squamous metaplasia of human limbal epithelium

PURPOSE

Squamous metaplasia is a pathologic process that frequently occurs in nonkeratinized stratified ocular surface epithelia. The mechanism for this occurrence is largely unknown except for vitamin A deficiency.

METHODS

Human limbal explants were cultured under airlift with or without p38 mitogen-activated protein kinase (MAPK) inhibitor SB203580 or in a submerged manner for different durations up to 2 weeks. Epithelial cell proliferation, differentiation, limbal stem cell maintenance, and expansion were studied using certain markers such as Ki67, p63, K10 and K12 keratins, filaggrin, Pax6, ABCG-2, and Musashi-1. Expression of phospho-p38 MAPK and its downstream transcription factors, C/EBPalpha and C/EBPbeta, were studied by immunohistochemistry. Epithelial cells harvested from explants after 2 weeks of culturing under different conditions were seeded onto 3T3 feeder layers and cultured for 12 days. The differentiation of clonal epithelial cells was investigated by double staining to K12 and K10 keratins.

RESULTS

The squamous metaplasia model was successfully created by culturing human limbal explants at an air-liquid interface (airlift) for 2 weeks. Increased stratification and hyperproliferation only happened in the limbal, but not the corneal, epithelium in airlift, but not submerged, cultures. Epithelial proliferation was associated with a transient increase of limbal epithelial stem cells. Abnormal epidermal differentiation-evidenced by positive expression of K10 keratin in suprabasal cells and filaggrin in superficial cells-ensued. Clones generated from epithelial cells harvested from airlift culture only expressed K12 keratin without K10. As early as 2 days in airlift cultures, p38 expression emerged in limbal basal epithelial cells and gradually extended to the cytoplasm and nuclei. Furthermore, addition of the p38 inhibitor SB203580 abolished abnormal epidermal differentiation without affecting limbal epithelial proliferation. Expression of C/EBPalpha and C/EBPbeta, downstream of the p38 MAPK signaling pathway, was strongly induced by airlift culture and partially was inhibited by SB203580.

CONCLUSIONS

Dryness resulting from exposure activates p38 MAPK signaling coupled with abnormal epidermal differentiation without intrinsic alteration of stem cells in the limbus. On the ocular surface, p38 inhibitors may have the potential to revert the pathologic process of squamous metaplasia induced by dryness.

Down-regulation of Pax6 is associated with abnormal differentiation of corneal epithelial cells in severe ocular surface diseases

Pax6 is the universal master control gene for eye morphogenesis. Other than retina and lens, Pax6 also expressed in the ocular surface epithelium from early gestation until the postnatal stage, in which little is known about the function of Pax6. In this study, corneal pannus tissues from patients with ocular surface diseases such as Stevens-Johnson syndrome (SJS), chemical burn, aniridia and recurrent pterygium were investigated. Our results showed that normal ocular surface epithelial cells expressed Pax6. However, corneal pannus epithelial cells from the above patients showed a decline or absence of Pax6 expression, accompanied by a decline or absence of K12 keratin but an increase of K10 keratin and filaggrin expression. Pannus basal epithelial cells maintained nuclear p63 expression and showed activated proliferation, evidenced by positive Ki67 and K16 keratin staining. On 3T3 fibroblast feeder layers, Pax6 immunostaining was negative in clones generated from epithelial cells harvested from corneal pannus from SJS or aniridia, but positive in those from the normal limbal epithelium; whereas western blots showed that some epithelial clones expanded from pannus retained Pax6 expression. Transient transfection of an adenoviral vector carrying EGFP-Pax6 transgenes into these Pax6(-) clones increased both Pax6 and K12 keratin expression. These results indicate that Pax6 helps to maintain the normal corneal epithelial phenotype postnatally, and that down-regulation of Pax6 is associated with abnormal epidermal differentiation in severe ocular surface diseases. Reintroduction of activation of the Pax6 gene might be useful in treating squamous metaplasia of the ocular surface epithelium.

A xenobiotic-free culture system for human limbal epithelial stem cells

AIMS

Murine 3T3 feeder cells are commonly used for stem cell expansion. Although 'feeder-free' systems are being developed for a variety of stem cells including embryonic, the use of feeder cells currently remains optimal for the expansion of epithelial stem cells. In this study, MRC-5, a human embryonic fibroblast cell line, has been investigated for its potential use as a feeder layer in human limbal epithelial (HLE) cell expansion under serum-free conditions, with the aim of developing a xenobiotic-free culture system for therapeutic corneal regeneration applications.

MATERIALS AND METHODS

MRC-5 feeder cells were compared with J2 3T3 mouse fibroblasts, in both serum-supplemented and serum-free conditions, in terms of their relative ability to support HLE cell metabolic activity, expression of the putative stem cell markers ABCG2 and P63 alpha, cell differentiation using the cornea-specific cytokeratin 3 antibody and colony-forming efficiency.

RESULTS

The proportion of HLE stem cells maintained was determined by functional colony-forming efficiency assays. The metabolic activity results showed that HLE cells cultured on MRC-5 fibroblasts under serum-free conditions proliferated as well as cells cultured on J2 cells under serum-free conditions. Moreover, the HLE cultured on MRC-5 fibroblasts under serum-free conditions expressed high levels of putative stem cell markers ABCG2 and P63 alpha and low levels of the differentiation marker CK3, indicating that they retained poorly differentiated 'stem cell-like' characteristics under those culture conditions. Clonal analysis of HLE cells cultured on growth-arrested feeder layers of J2 and MRC-5 fibroblasts showed that cells expanded on MRC-5 and J2 fibroblasts in serum-free conditions had a colony-forming efficiency of approximately 1.5%, indicating the maintenance of stem cells.

CONCLUSIONS

These results demonstrate feasibility of expanding HLE cells for clinical purposes by using a human fibroblast cell line as a feeder layer, avoiding the use of bovine serum, while preserving the proliferative potential and stem cell characteristics of HLE cells.

Side population cells in the human vocal fold

OBJECTIVES

The regenerative processes of the vocal fold, or the existence of stem cells in the folds, are unknown. Side population (SP) cells are defined as cells that have the ability to exclude the DNA binding dye, Hoechst 33342. They are regarded as a cell population enriched with stem cells and can be isolated from non-SP cells by a fluorescence-activated cell sorter. This study was designed to determine whether SP cells exist in the human vocal fold, as a first step in elucidating the regenerative mechanisms of the vocal fold.

METHODS

Seven human excised larynges were used in this study. Two were used for fluorescence-activated cell sorter analysis, and 5 were subjected to immunohistochemical analysis with antibodies against an adenosine triphosphate binding cassette transporter family member, ABCG2, which is expressed in SP cells.

RESULTS

The number of SP cells in the human vocal fold was about 0.2% of the total number of cells. ABCG2-positive cells were identified in both the epithelium and subepithelial tissue throughout the entire vocal fold.

CONCLUSIONS

This preliminary study demonstrated the existence of SP cells in the human vocal fold. Further studies are warranted to clarify how these cells work in the vocal fold, particularly in the regenerative process.

Patterned expression of neurotrophic factors and receptors in human limbal and corneal regions

PURPOSE

To evaluate the expression patterns of neurotrophic factors (NTFs) and their receptors in the human cornea with the intention of exploring the role of NTFs in maintaining corneal epithelial stem cells in the limbus.

METHODS

Fresh human corneoscleral tissues were prepared for frozen sections. Immunofluorescent staining was performed with primary antibodies against six members of three NTF families and their six receptors. To confirm the specificity of NTF primary antibodies, neutralization experiments with their corresponding peptides and western blot analysis were performed.

RESULTS

Based on spatial and differential immuno-localization, three patterns of NTF expression were potentially involved in epithelial-mesenchymal interaction on the ocular surface: (1) the epithelial type: nerve growth factor (NGF) and glial cell-derived neurotrophic factor (GDNF); (2) the paracrine type: neurotrophin (NT)-3 and NT-4/5; and (3) the reciprocal type: brain-derived neurotrophic factor (BDNF). The stem cell-enriched basal cells of the limbal epithelium expressed three unique staining patterns for NTFs: (1) exclusively positive for NGF, GDNF, and their corresponding receptors, TrkA and GDNF family receptor alpha (GFRalpha)-1; (2) relatively high levels of BDNF; and (3) negative for NT-3 and NT-4. Additionally, the neurotrophin common low-affinity receptor, p75NTR, was mainly expressed by the basal layer of the entire corneal and limbal epithelia, and TrkB and TrkC were evenly expressed by the entire corneal and limbal epithelia. BDNF, p75NTR, TrkB, and TrkC are also abundantly expressed by limbal stroma cells. No specific immunoreactivity to ciliary neurotrophic factor (CNTF) and its receptor, CNTFRalpha, was detected in cornea tissue in situ.

CONCLUSIONS

Our findings revealed patterned expression of NTFs and their receptors in the human ocular surface, suggesting that they may play a vital role in maintaining corneal epithelial stem cells in the limbus. NGF, GDNF, GFRalpha-1, TrkA, and BDNF may serve as new limbal basal cell markers defining the corneal epithelial stem cell phenotype.

Transplantation of ex vivo cultured limbal epithelial stem cells: a review of techniques and clinical results

Ex vivo cultured limbal epithelial stem cells have been used successfully to treat corneal limbal stem cell deficiency. We identified 17 reports of the application of this novel cell-based therapy in humans. In addition we identified four reports of the use of culture oral mucosal epithelial cells to treat limbal stem cell deficiency. We examined these reports to discern the success rate, complication rate, visual outcome, whether there is an optimal technique and which patients are the most likely to benefit. We also discuss the different culture methods employed and the regulations governing cell banks that are providing this service. We found that the techniques used to cultivate and transplant cells varied, but that no individual method was clearly superior. The reported success rate is similar across all studies for both allografts and autografts. The clinical indications for this treatment are not clearly defined as indicated by the variety of disorders treated. Follow-up is limited and the long-term success rate is yet to be established. Nonetheless, we conclude that there is sufficient evidence to support the continued use and refinement of this procedure as a treatment for corneal stem cell deficiency.

Side population cells expressing ABCG2 in human adult dental pulp tissue

AIM

To investigate the presence of side population (SP) cells by the Hoechst exclusion method in human adult dental pulp tissue.

METHODOLOGY

Human adult dental pulp-derived cells were generated from third molar teeth. The cells were stained with Hoechst 33342 and sorted into SP cells or non-SP cells [main population (MP) cells]. Both cell types were compared with cell growth and RT-PCR analyses.

RESULTS

SP cells that express ABCG2, Nestin, Notch-1 and alpha-smooth muscle actin were found at frequencies ranging from 0.67% to 1.02%. This SP profile disappeared in the presence of verapamil. These SP cells expressed dentine sialophosphoprotein and dentine matrix protein-1 when cultured in osteogenic medium.

CONCLUSION

Human adult dental pulp tissue contains SP cells that differentiate into odontoblast-like cells.

Nerve growth factor and its receptor TrkA serve as potential markers for human corneal epithelial progenitor cells

Nerve growth factor (NGF), a member of the neurotrophin family, has been identified as an essential growth factor supporting stem cell self-renewal outside the nervous system and was previously shown to stimulate corneal epithelial proliferation both in vivo and in vitro. In this study, we evaluated the expression of NGF and its corresponding receptors in the human corneal and limbal tissues, as well as in primary limbal epithelial cultures by immunofluorescent staining and relatively quantitative real-time polymerase chain reaction. We found that NGF was uniquely expressed in the human limbal basal epithelium, together with its two corresponding receptors: the high-affinity receptor TrkA and the low-affinity receptor p75NTR. TrkA was shown to preferentially localize to limbal basal epithelial cells. NGF and TrkA were also found co-localized with stem cell-associated molecular markers (drug-resistance transporter ABCG2 and p63), but not with the differentiation marker cytokeratin 3 in the human limbal basal epithelial layer. In cultured limbal epithelial cells, NGF and TrkA were found to be preferentially expressed by a small population of limbal epithelial cells. The NGF and TrkA immuno-positive subpopulations were enriched for certain properties (including ABCG2 and p63 expression) of putative limbal epithelial stem cells (P<0.01, compared with the entire cell population). Levels of NGF and TrkA transcripts were found to be much more abundant in limbal than in corneal tissues, and in young cultured cells in the proliferative stage than in airlifted stratified cultures containing differentiated cells. The co-expression of NGF with its two corresponding receptors in limbal basal epithelial cells, but not in the cornea, suggests that NGF may function as a critical autocrine or paracrine factor supporting stem cell self-renewal in the limbal stem cell niche. The spatial expression of NGF and TrkA by small clusters of basal cells interspersed between negative cell patches suggests that they are potential markers for human corneal epithelial progenitor cells.

Mitral valvular interstitial cells demonstrate regional, adhesional, and synthetic heterogeneity

BACKGROUND/AIMS

Because various regions of the mitral valve contain distinctive extracellular matrix enabling the tissues to withstand diverse mechanical environments, we investigated phenotype and matrix production of porcine valvular interstitial cells (VICs) from different regions.

METHODS

VICswere isolated from the chordae (MCh), the center of the anterior leaflet (AlCtr), and the posterior leaflet free edge (PlFree), then assayed for metabolic, growth, and adhesion rates; collagen and glycosaminoglycan (GAG) production, and phenotype using biochemical assays, flow cytometry, and immunocytochemistry.

RESULTS

The AlCtr VICs exhibited the fastest metabolism but slowest growth. PlFree cells grew the fastest, but demonstrated the least smooth muscle alpha-actin, vimentin, and internal complexity. AlCtr VICs secreted less collagen into the culture medium but more 4-sulfated GAGs than other cells. Adhesion-based separation resulted in altered secretion of sulfated GAGs by MCh and AlCtr cells but not by the PlFree cells.

CONCLUSIONS

VICs isolated from various regions of the mitral valve demonstrate phenotypic differences in culture, corresponding to the ability of the mitral valve to accommodate the physical stresses or altered hemodynamics that occur with injury or disease. Further understanding of VIC and valve mechanobiology could lead to novel medical or tissue engineering approaches to treat valve diseases.

Epithelial stem cells of the eye surface

OBJECTIVES

Epithelial stem cells of the eye surface, of the cornea and of the conjunctiva, have the ability to give rise to self renewal and progeny production of differentiated cells with no apparent limit. The two epithelia are separated from each other by the transition zone of the limbus. The mechanisms adopted by stem cells of the two epithelia to accomplish their different characteristics, and how their survival, replacement and unequal division that generates differentiated progeny formation are controlled, are complex and still poorly understood. They can be learned only by understanding how stem cells/progenitors are regulated by their neighbouring cells, that may themselves be differently unspecialised, forming particular microenvironments, known as 'niches'. Stem cells operate by signals and a variety of intercellular interactions and extracellular substrates with adjacent cells in the niche. Technical advances are now making it possible to identify zones in the corneal limbus and conjunctiva that can house stem cells, to isolate and expand them ex vivo and to control their behaviour creating optimal niche conditions. With improvements in biotechnology, regenerative cornea and conjunctiva transplantation using adult epithelial stem cells becomes now a reality.

RESULTS AND CONCLUSIONS

Here we review our current understanding of stem cell niches and illustrate recent significant progress for identification and characterization of adult epithelial stem cells/progenitors at cellular, molecular and mechanistic levels, improvement in cell culture techniques for their selective expansion ex vivo and prospects for a variety of therapeutic applications.

Maintenance of retinal stem cells by Abcg2 is regulated by notch signaling

ABCG2 belongs to the ATP-binding cassette superfamily of transmembrane proteins and is ubiquitously expressed in stem cells including those in the developing nervous system. The ability of ABCG2 to preferentially exclude DNA-intercalating dyes is regarded to be the basis for the enrichment of stem cells or progenitors as dye(low) side population (SP) cells. However, the role of ABCG2 in neural stem cells remains speculative and poorly understood. Here, we demonstrate using retinal stem cells, that ABCG2 is the molecular determinant of SP cell phenotype of neural stem cells and plays an important role in their maintenance. Overexpression of ABCG2 prevents the SP cell phenotype and adversely affects the lineage commitment of retinal stem cells. By contrast, targeted attenuation of ABCG2 depletes retinal SP cells and promotes their differentiation along pan neural and retinal lineages. In addition, we demonstrate for the first time that ABCG2 is a target of Notch signaling, and as such, constitutes one of the genes in the regulatory network of Notch signaling, involved in the maintenance of stem cells.

ABCG2: determining its relevance in clinical drug resistance

Multidrug resistance is a major obstacle to successful cancer treatment. One mechanism by which cells can become resistant to chemotherapy is the expression of ABC transporters that use the energy of ATP hydrolysis to transport a wide variety of substrates across the cell membrane. There are three human ABC transporters primarily associated with the multidrug resistance phenomenon, namely Pgp, MRP1, and ABCG2. All three have broad and, to a certain extent, overlapping substrate specificities, transporting the major drugs currently used in cancer chemotherapy. ABCG2 is the most recently described of the three major multidrug-resistance pumps, and its substrates include mitoxantrone, topotecan, irinotecan, flavopiridol, and methotrexate. Despite several studies reporting ABCG2 expression in normal and malignant tissues, no trials have thus far addressed the role of ABCG2 in clinical drug resistance. This gives us an opportunity to critically review the disappointing results of past clinical trials targeting Pgp and to propose strategies for ABCG2. We need to know in which tumor types ABCG2 contributes to the resistance phenotype. We also need to develop standardized assays to detect ABCG2 expression in vivo and to carefully select the chemotherapeutic agents and clinical trial designs. This review focuses on our current knowledge about normal tissue distribution, tumor expression profiles, and substrates and inhibitors of ABCG2, together with lessons learned from clinical trials with Pgp inhibitors. Implications of SNPs in the ABCG2 gene affecting the pharmacokinetics of substrate drugs, including many non-chemotherapy agents and ABCG2 expression in the SP population of stem cells are also discussed.

Comparative analysis of human conjunctival and corneal epithelial gene expression with oligonucleotide microarrays

PURPOSE

To determine global mRNA expression levels in corneal and conjunctival epithelia and identify transcripts that exhibit preferential tissue expression.

METHODS

cDNA samples derived from human conjunctival and corneal epithelia were hybridized in three independent experiments to a commercial oligonucleotide array representing more than 22,000 transcripts. The resultant signal intensities and microarray software transcript present/absent calls were used in conjunction with the local pooled error (LPE) statistical method to identify transcripts that are preferentially or exclusively expressed in one of the two tissues at significant levels (expression >1% of the beta-actin level). EASE (Expression Analysis Systematic Explorer software) was used to identify biological systems comparatively overrepresented in either epithelium. Immuno-, and cytohistochemistry was performed to validate or expand on selected results of interest.

RESULTS

The analysis identified 332 preferential and 93 exclusive significant corneal epithelial transcripts. The corresponding numbers of conjunctival epithelium transcripts were 592 and 211, respectively. The overrepresented biological processes in the cornea were related to cell adhesion and oxiredox equilibria and cytoprotection activities. In the conjunctiva, the biological processes that were most prominent were related to innate immunity and melanogenesis. Immunohistochemistry for antigen-presenting cells and melanocytes was consistent with these gene signatures. The transcript comparison identified a substantial number of genes that have either not been identified previously or are not known to be highly expressed in these two epithelia, including testican-1, ECM1, formin, CRTAC1, and NQO1 in the cornea and, in the conjunctiva, sPLA(2)-IIA, lipocalin 2, IGFBP3, multiple MCH class II proteins, and the Na-Pi cotransporter type IIb.

CONCLUSIONS

Comparative gene expression profiling leads to the identification of many biological processes and previously unknown genes that are potentially active in the function of corneal and conjunctival epithelia.

Niche regulation of corneal epithelial stem cells at the limbus

Among all adult somatic stem cells, those of the corneal epithelium are unique in their exclusive location in a defined limbal structure termed Palisades of Vogt. As a result, surgical engraftment of limbal epithelial stem cells with or without ex vivo expansion has long been practiced to restore sights in patients inflicted with limbal stem cell deficiency. Nevertheless, compared to other stem cell examples, relatively little is known about the limbal niche, which is believed to play a pivotal role in regulating self-renewal and fate decision of limbal epithelial stem cells. This review summarizes relevant literature and formulates several key questions to guide future research into better understanding of the pathogenesis of limbal stem cell deficiency and further improvement of the tissue engineering of the corneal epithelium by focusing on the limbal niche.

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.

Limbal stem cells, a review of their identification and culture for clinical use

The surface of the eye is covered by two distinct epithelial populations, the conjunctival and corneal epithelia. The stem cell population for the corneal epithelia has been found to be located at the area known as the limbus. This is a narrow ring of tissue at the transitional zone between the cornea and conjunctiva. This stem cell population is responsible for generating transient amplifying cells which are responsible for renewing the cornea epithelia. There are currently no definitive markers for the stem cell population in the limbus. Instead using morphological features, such as small cells with a high nucleus-to-cytoplasm ratio, in conjunction with the presence of certain markers e.g. DeltaNP63alpha and the absence of others, e.g. the cytokeratin pair 3 & 12, are taken as being indicative of the stem cell population. Damage can occur to the corneal epithelium due to a number of causes including, Steven-Johnson syndrome, and chemical or thermal burns. This results in invasion of the cornea by the conjunctival epithelium resulting in impaired vision. In 1997 Pellegrini et al. (Lancet 349, 990) successfully used cells sheets from cultured limbal cells to successfully treat patients with corneal damage. Since then several other groups, have successfully treated patients, using similar methods.