Hypoxia is a key regulator of limbal epithelial stem cell growth and differentiation

Effects of hypoxia in the diabetic corneal stroma microenvironment

Corneal dysfunctions associated with Diabetes Mellitus (DM), termed diabetic keratopathy (DK), can cause impaired vision and/or blindness. Hypoxia affects both Type 1 (T1DM) and Type 2 (T2DM) surprisingly, the role of hypoxia in DK is unexplored. The aim of this study was to examine the impact of hypoxia in vitro on primary human corneal stromal cells derived from Healthy (HCFs), and diabetic (T1DMs and T2DMs) subjects, by exposing them to normoxic (21% O2) or hypoxic (2% O2) conditions through 2D and 3D in vitro models. Our data revealed that hypoxia affected T2DMs by slowing their wound healing capacity, leading to significant alterations in oxidative stress-related markers, mitochondrial health, cellular homeostasis, and endoplasmic reticulum health (ER) along with fibrotic development. In T1DMs, hypoxia significantly modulated markers related to membrane permeabilization, oxidative stress via apoptotic marker (BAX), and protein degradation. Hypoxic environment induced oxidative stress (NOQ1 mediated reduction of superoxide in T1DMs and Nrf2 mediated oxidative stress in T2DMs), modulation in mitochondrial health (Heat shock protein 27 (HSP27), and dysregulation of cellular homeostasis (HSP90) in both T1DMs and T2DMs. This data underscores the significant impact of hypoxia on the diabetic cornea. Further studies are warranted to delineate the complex interactions.

The progress in techniques for culturing human limbal epithelial stem cells

In vitro culture of human limbal epithelial stem cells (hLESCs) is crucial to cell therapy in the treatment of limbal stem cell deficiency, a potentially vision-threatening disease that is characterized by persistent corneal epithelial defects and corneal epithelium conjunctivalization. Traditionally, hLESCs are cultivated based on either limbal tissue explants or single-cell suspensions in culture media containing xenogenous components, such as fetal bovine serum and murine 3T3 feeder cells. Plastic culture dishes and human amniotic membranes are classical growth substrates used in conventional hLESC culture systems. The past few decades have witnessed considerable progress and innovations in hLESC culture techniques to ensure a higher level of biosafety and lower immunogenicity for further cell treatment, including complete removal of xenogenous components from culture media, the application of human-derived feeder cells, and the development of novel scaffolds. Three-dimensional artificial niches and three-dimensional culture techniques have also been established to simulate the real microenvironment of limbal crypts for better cell outgrowth and proliferation. All these progresses ensure that in vitro cultured hLESCs are more adaptable to translational stem cell therapy for limbal stem cell deficiency.

Distribution of Stromal Cell Subsets in Cultures from Distinct Ocular Surface Compartments

Purpose

To reveal the phenotypic differences between human ocular surface stromal cells (hOSSCs) cultured from the corneal, limbal, and scleral compartments.

Methods

A comparative analysis of cultured hOSSCs derived from four unrelated donors was conducted by multichromatic flow cytometry for six distinct CD antigens, including the CD73, CD90, CD105, CD166, CD146, and CD34.

Results

The hOSSCs, as well as the reference cells, displayed phenotypical profiles that were similar in high expression of the hallmark mesenchymal stem cell markers CD73, CD90, and CD105, and also the cancer stem cell marker CD166. Notably, there was considerable variation regarding the expression of CD34, where the highest levels were found in the corneal and scleral compartments. The multi-differentiation potential marker CD146 was also expressed highly variably, ranging from 9% to 89%, but the limbal stromal and endometrial mesenchymal stem cells significantly surpassed their counterparts within the ocular and reference groups, respectively. The use of six markers enabled investigation of 64 possible variants, however, just four variants accounted for almost 90% of all hOSSCs, with the co-expression of CD73, CD90, CD105, and CD166 and a combination of CD146 and CD34. The limbal compartment appeared unique in that it displayed greatest immunophenotype diversity and harbored the highest proportion of the CD146+CD34- pericyte-like forms, but, interestingly, the pericyte-like cells were also found in the avascular cornea.

Conclusion

Our findings confirm that the hOSSCs exhibit an immunophenotype consistent with that of MSCs, further highlight the phenotypical heterogeneity in stroma from distinct ocular surface compartments, and finally underscore the uniqueness of the limbal region.

Novelty in limbal stem cell culture and cell senescence

Limbal stem cell deficiency is a pathological state. Recently, limbal stem cell (LSC) transplantation has attracted great interest as a therapeutic method which mainly involves in-vitro expansion of LSCs. It is believed that ex-vivo cultivation conditions could affect the outcome of surgery and the rate of successful transplantation. Thus, we aimed to define a suitable culture condition by conducting a research on ex-vivo expanded LSCs to maintain an optimized graft of amniotic membrane with cultivated-limbal stem cells, regarding the quantity and quality, with the hope of improving the clinical outcome.

Pigmentation Is Associated with Stemness Hierarchy of Progenitor Cells Within Cultured Limbal Epithelial Cells

Ex vivo cultured human limbal epithelial stem/progenitor cells (hLESCs) are the main source for regenerative therapy of limbal stem cell deficiency (LSCD), which is worldwide one of the major causes of corneal blindness. Despite many stemness-associated markers have been identified within the limbal niche, the phenotype of the earliest hLESCs has not been hitherto identified. We sought to confirm or refute the use of tumor protein p63 (p63) and ATP binding cassette subfamily B member 5 (ABCB5) as surrogate markers for hLESCs early within the limbal differentiation hierarchy. Based on a robust fluorescence-activated cell sorting and subsequent RNA isolation protocol, a comprehensive transcriptomic profile was obtained from four subpopulations of cultured hLESCs. The subpopulations were defined by co-expression of two putative stem/progenitor markers, the p63 and ABCB5, and the corneal differentiation marker cytokeratin 3. A comparative transcriptomic analysis yielded novel data that indicated association between pigmentation and differentiation, with the p63 positive populations being the most pigmented and immature of the progenitors. In contrast, ABCB5, either alone or in co-expression patterns, identified more committed progenitor cells with less pigmentation. In conclusion, p63 is superior to ABCB5 as a marker for stemness. Stem Cells 2018;36:1411-1420.

Hypoxia Signaling Pathway in Stem Cell Regulation: Good and Evil

Purpose of Review

This review summarizes the role of hypoxia and hypoxia-inducible factors (HIFs) in the regulation of stem cell biology, specifically focusing on maintenance, differentiation, and stress responses in the context of several stem cell systems. Stem cells for different lineages/tissues reside in distinct niches, and are exposed to diverse oxygen concentrations. Recent studies have revealed the importance of the hypoxia signaling pathway for stem cell functions.

Recent Findings

Hypoxia and HIFs contribute to maintenance of embryonic stem cells, generation of induced pluripotent stem cells, functionality of hematopoietic stem cells, and survival of leukemia stem cells. Harvest and collection of mouse bone marrow and human cord blood cells in ambient air results in fewer hematopoietic stem cells recovered due to the phenomenon of Extra PHysiologic Oxygen Shock/Stress (EPHOSS).

Summary

Oxygen is an important factor in the stem cell microenvironment. Hypoxia signaling and HIFs play important roles in modeling cellular metabolism in both stem cells and niches to regulate stem cell biology, and represent an additional dimension that allows stem cells to maintain an undifferentiated status and multilineage differentiation potential.

Maintaining RNA Integrity for Transcriptomic Profiling of Ex Vivo Cultured Limbal Epithelial Stem Cells after Fluorescence-Activated Cell Sorting (FACS)

Background

Transcriptomic profiling of ex vivo cultured human limbal epithelial stem cells (hLESCs) will foster better understanding of corneal physiology and novel treatment paradigms to limbal stem cell deficiency (LSCD). However, currently such profiling studies are hampered due to difficulties with producing sufficient amounts of intact mRNA for deep RNA sequencing (RNA-seq) from subpopulations sorted on the basis of co-expression of membrane and intracellular antigens by fluorescence-activated cell sorting (FACS).

Methods

To address this problem, we systematically analyzed the critical steps, and found that ethanol fixation together with optimized downstream procedures provided a pipeline that yielded high quality total RNA in amounts to readily support the RNA-seq procedure, while still preserving good discrimination between the individual hLESC immunophenotypes.

Results

The average RNA integrity number (RIN) was 7.7 ± 0.4, and the average yield was 4.6 ± 1.7 pg of RNA per cell. The sequencing analysis of the isolated RNA produced high quality data with more than 70% of read pairs mapping uniformly to the reference genome and 80% of bases with a Phred score of at least 30.

Conclusion

In this study, we developed a reliable FACS-based procedure using ethanol as a fixative that would support accurate isolation of limbal epithelial progenitor subpopulations along with RNA yield and quality sufficient to enable deep transcriptomic profiling.

Low intensity transcranial electric stimulation: Safety, ethical, legal regulatory and application guidelines

Low intensity transcranial electrical stimulation (TES) in humans, encompassing transcranial direct current (tDCS), transcutaneous spinal Direct Current Stimulation (tsDCS), transcranial alternating current (tACS), and transcranial random noise (tRNS) stimulation or their combinations, appears to be safe. No serious adverse events (SAEs) have been reported so far in over 18,000 sessions administered to healthy subjects, neurological and psychiatric patients, as summarized here. Moderate adverse events (AEs), as defined by the necessity to intervene, are rare, and include skin burns with tDCS due to suboptimal electrode-skin contact. Very rarely mania or hypomania was induced in patients with depression (11 documented cases), yet a causal relationship is difficult to prove because of the low incidence rate and limited numbers of subjects in controlled trials. Mild AEs (MAEs) include headache and fatigue following stimulation as well as prickling and burning sensations occurring during tDCS at peak-to-baseline intensities of 1-2mA and during tACS at higher peak-to-peak intensities above 2mA. The prevalence of published AEs is different in studies specifically assessing AEs vs. those not assessing them, being higher in the former. AEs are frequently reported by individuals receiving placebo stimulation. The profile of AEs in terms of frequency, magnitude and type is comparable in healthy and clinical populations, and this is also the case for more vulnerable populations, such as children, elderly persons, or pregnant women. Combined interventions (e.g., co-application of drugs, electrophysiological measurements, neuroimaging) were not associated with further safety issues. Safety is established for low-intensity 'conventional' TES defined as <4mA, up to 60min duration per day. Animal studies and modeling evidence indicate that brain injury could occur at predicted current densities in the brain of 6.3-13A/m2 that are over an order of magnitude above those produced by tDCS in humans. Using AC stimulation fewer AEs were reported compared to DC. In specific paradigms with amplitudes of up to 10mA, frequencies in the kHz range appear to be safe. In this paper we provide structured interviews and recommend their use in future controlled studies, in particular when trying to extend the parameters applied. We also discuss recent regulatory issues, reporting practices and ethical issues. These recommendations achieved consensus in a meeting, which took place in Göttingen, Germany, on September 6-7, 2016 and were refined thereafter by email correspondence.

Insulin Growth Factor 1 Protects Neural Stem Cells Against Apoptosis Induced by Hypoxia Through Akt/Mitogen-Activated Protein Kinase/Extracellular Signal-Regulated Kinase (Akt/MAPK/ERK) Pathway in Hypoxia-Ishchemic Encephalopathy

Background

Insulin growth factor 1 (IGF-1) is reported to modulate cell growth and acts as potential therapy for traumatic brain injury. This study was designed to investigate the effect of IGF-1 on hypoxia-induced apoptosis in neural stem cells (NSCs).

Material/Methods

A hypoxia model was constructed using NSCs separated from the hippocampus of rat. NSCs were divided into four groups: cells under normoxic conditions that acted as controls (C group), cells under hypoxia (H group), cells under hypoxia with IGF-1 (HI group), and cells under hypoxia with IGF-1 as well as picropodophyllin (PPP), which acts as an inhibitor of the IGF-1 receptor (HIP group). The cell viability and apoptosis were respectively measured by 3-(4,5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) assay and flow cytometry. Finally, the phosphorylation levels of apoptosis-associated proteins and key kinases in the phosphatidylinositol-3-kinase (PI3K)/AKT and the mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) pathways were detected by Western blot analysis.

Results

In comparison with the H group, the cell viability was increased while the cell apoptosis was reduced by IGF-1 in the HI group. Besides, the expression levels of Bax, cytochrome c, and activated caspase-3 were all improved in the H group, and the remarkable differences were eliminated in the HI group compared with the C group. The expression level of Bcl-2 was the opposite. Additionally, down-regulations of phosphorylated AKT, MAPK, and ERK induced by hypoxia were all improved by IGF-1. All the influences of IGF-1 were weakened by addition of PPP.

Conclusions

IGF-1 increased cell viability while decreasing apoptosis in hypoxic NSCs through the PI3K/AKT and MAPK/ERK pathways.

Hypoxia induces an undifferentiated phenotype of oral keratinocytes in vitro

The aim of this study was to determine the effects of hypoxia on the proliferating potential and phenotype of primary human oral keratinocytes cultured at ambient oxygen tension (20%) or at different levels of hypoxia (2 and 0.5% O2). The effects of oxygen tensions on cellular metabolic activity, cell proliferation, clonogenicity and proliferation heterogeneity were measured. Cell cycle profiles were analyzed by a fluorescent-activated cell sorter, and p21(WAF1/CIP1) expression in the G0/G1 phase was also concomitantly quantitated. The expression levels of cell cycle regulatory proteins were examined by immunoblotting, and the cellular senescence was assessed by senescence-associated β-galactosidase staining. Basal and suprabasal keratinocyte phenotypes were determined by the expression levels of 14-3-3σ, p75(NTR) and α6 integrin. Despite having a lower metabolism, the proliferation rate and clonogenic potential were remarkably enhanced in hypoxic cells. The significantly higher percentage of cells in the G0/G1 phase under hypoxia and the expression patterns of cell cycle regulatory proteins in hypoxic cells were indicative of a state of cell cycle arrest in hypoxia. Furthermore, a decrease in the expression of p21(WAF1/CIP1) and p16(INK4A) and fewer β-galactosidase-positive cells suggested a quiescent phenotype rather than a senescent one in hypoxic cells. Compared with normoxic cells, the differential expression patterns of keratinocyte phenotypic markers suggest that hypoxic cells that generate minimal reactive oxygen species, suppress the mammalian target of rapamycin activity and express hypoxia-inducible factor-1α favor a basal cell phenotype. Thus, regardless of the predisposition to the state of cell cycle arrest, hypoxic conditions can maintain oral keratinocytes in vitro in an undifferentiated and quiescent state.

Upregulation of P-glycoprotein expression by ophthalmic drugs in different corneal in-vitro models

OBJECTIVES

The purpose of this study was to analyse P-glycoprotein (P-gp) expression in different human in-vitro cornea models (HCE-T epithelial model and Hemicornea construct) after stimulation with P-gp substrates (rhodamine 123, levofloxacin and acebutolol).

METHODS

The influence of P-gp substrates on mRNA expression was analysed using reverse transcriptase polymerase chain reaction (PCR) and real-time PCR. The effect of stimulation on the transporter functionality was estimated with a digoxin efflux assay. The Caco-2 cell line was used as positive control.

KEY FINDINGS

The reverse transcriptase PCR results showed an increase in band intensity compared with the control medium for all substrates. The real-time PCR for the Caco-2 and HCE-T epithelial model yielded a similar outcome, in which all tested substrates upregulated P-gp. In contrast, the Hemicornea construct showed no significant increase in the mRNA expression after stimulation. Both in-vitro models possessed similar drug transport profiles after stimulation. A significantly increased efflux of digoxin was measured after 24 and 72 h of stimulation with levofloxacin and acebutolol.

CONCLUSIONS

The expression and functionality of the P-gp in corneal tissue can be influenced through time exposure with specific substrates. However, the exact mechanism still requires further elucidation.

Spatiotemporal oxygen sensing using dual emissive boron dye-polylactide nanofibers

Oxygenation in tissue scaffolds continues to be a limiting factor in regenerative medicine despite efforts to induce neovascularization or to use oxygen-generating materials. Unfortunately, many established methods to measure oxygen concentration, such as using electrodes, require mechanical disturbance of the tissue structure. To address the need for scaffold-based oxygen concentration monitoring, a single-component, self-referenced oxygen sensor was made into nanofibers. Electrospinning process parameters were tuned to produce a biomaterial scaffold with specific morphological features. The ratio of an oxygen sensitive phosphorescence signal to an oxygen insensitive fluorescence signal was calculated at each image pixel to determine an oxygenation value. A single component boron dye-polymer conjugate was chosen for additional investigation due to improved resistance to degradation in aqueous media compared to a boron dye polymer blend. Standardization curves show that in fully supplemented media, the fibers are responsive to dissolved oxygen concentrations less than 15 ppm. Spatial (millimeters) and temporal (minutes) ratiometric gradients were observed in vitro radiating outward from the center of a dense adherent cell grouping on scaffolds. Sensor activation in ischemia and cell transplant models in vivo show oxygenation decreases on the scale of minutes. The nanofiber construct offers a robust approach to biomaterial scaffold oxygen sensing.

Creating the bioartificial myocardium for cardiac repair: challenges and clinical targets

The association of stem cells with tissue-engineered scaffolds constitutes an attractive approach for the repair of myocardial tissue with positive effects to avoid ventricular chamber dilatation, which changes from a natural elliptical to spherical shape in heart failure patients. Biohybrid scaffolds using nanomaterials combined with stem cells emerge as new therapeutic tool for the creation of 'bioartificial myocardium' and 'cardiac wrap bioprostheses' for myocardial regeneration and ventricular support. Biohybrids are created introducing stem cells and self-assembling peptide nanofibers inside a porous elastomeric membrane, forming cell niches. Our studies lead to the creation of semi-degradable 'ventricular support bioprostheses' for adaptative LV and/or RV wrapping, designed with the concept of 'helical myocardial bands'. The goal is to restore LV elliptical shape, and contribute to systolic contraction and diastolic filling (suction mechanism). Cardiac wrapping with ventricular bioprostheses may reduce the risk of heart failure progression and the indication for heart transplantation.

Optimal isolation and xeno-free culture conditions for limbal stem cell function

PURPOSE

To preserve limbal stem cell (LSC) function in vitro with xenobiotic-free culture conditions.

METHODS

Limbal epithelial cells were isolated from 139 donors using 15 variations of three dissociation solutions. All culture conditions were compared to the baseline condition of murine 3T3-J3 feeders with xenobiotic (Xeno) keratinocyte growth medium at 20% O2. Five Xeno and Xeno-free media with increasing concentrations of calcium and epidermal growth factor (EGF) were evaluated at 5%, 14%, and 20% O2. Human MRC-5, dermal (fetal, neonatal, or adult), and limbal stromal fibroblasts were compared. Statistical analysis was performed on the number of maximum serial weekly passages, percentage of aborted colonies, colony-forming efficiency (CFE), p63α(bright) cells, and RT-PCR ratio of p63α/K12. Immunocytochemistry and RT-PCR for p63α, ABCG2, Bmi1, C/EBPδ , K12, and MUC1 were performed to evaluate phenotype.

RESULTS

Dispase/TrypLE was the isolation method that consistently showed the best yield, viability, and CFE. On 3T3-J2 feeders, Xeno-free medium with calcium 0.1 mM and EGF 10 ng/mL at 20% O2 supported more passages with equivalent percentage of aborted colonies, p63α(bright) cells, and p63α/K12 RT-PCR ratio compared to baseline Xeno-media. With this Xeno-free medium, MRC-5 feeders showed the best performance, followed by fetal, neonatal, adult HDF, and limbal fibroblasts. MRC-5 feeders supported serial passages with sustained high expression of progenitor cell markers at levels as robust as the baseline condition without significant difference between 20% and 5% O2.

CONCLUSIONS

The LSC function can be maintained in vitro under appropriate Xeno-free conditions.

Human corneal epithelial subpopulations: oxygen dependent ex vivo expansion and transcriptional profiling

Corneal epithelium is being regenerated throughout life by limbal epithelial stem cells (LESCs) believed to be located in histologically defined stem cell niches in corneal limbus. Defective or dysfunctional LESCs result in limbal stem cell deficiency (LSCD) causing pain and decreased visual acuity. Since the first successful treatment of LSCD by transplantation of ex vivo expanded LESCs in 1997, many attempts have been carried out to optimize culture conditions to improve the outcome of surgery. To date, progress in this field of bioengineering is substantially hindered by both the lack of specific biomarkers of LESCs and the lack of a precise molecular characterization of in situ epithelial subpopulations. The aim of this dissertation was to optimize culture systems with regard to the environmental oxygen concentration for selective ex vivo expansion of LESCs and to analyse in situ subpopulations in human corneal epithelium using a combination of laser capture microdissection and RNA sequencing for global transcriptomic profiling. We compared dissociation cultures, using either expansion on γ-irradiated NIH/3T3 feeder cells in serum-rich medium or expansion directly on plastic in serum-free EpiLife medium, using a range of physiologically relevant oxygen concentrations (2%, 5%, 10%, 15% and 20%). Using immunocytochemistry and advanced fluorescence microscopy, cells were characterized regarding growth, cell cycle distribution, colony-forming efficiency (CFE), phenotypes and cytomorphometry. Limbal epithelial cells expanded in 2% O2 exhibited slow growth, low fraction of cells in S/G2 , high CFE, high expression of stem cell markers ABCG2 and p63α, and low fraction of differentiation marker CK3 resembling a LESC phenotype. The effect of hypoxia to maintain LESCs in culture was not dependent on the system used for propagation (Bath et al. 2013a). Laser capture microdissection was used to isolate cellular subpopulations in situ from the spatially defined differentiation pathway in human corneal epithelium according to an optimized protocol for maintenance of expression profiles. Isolated total RNA from basal limbal crypts (BLCs), superficial limbal crypts (SLCs), paracentral/central cornea and limbal stroma was amplified and converted to fragmented cDNA libraries for use in deep paired-end next-generation sequencing. Global transcriptional profiling was carried out using bioinformatics. The location of primitive cells in BLCs, migratory and activated cells in SLCs and differentiated cells in paracentral/central cornea was evident from mapping of significantly upregulated genes in each compartment to the gene ontology (GO). Interestingly, many GO terms in BLCs were also involved in neurogenic processes, whereas many GO terms in SLCs were related to vasculature. Mapping upregulated genes in BLCs to pathway annotations in Kyoto Encyclopedia of Genes and Genomes described many active pathways as signalling and cancer-associated pathways. We supply extensive information on possible novel biomarkers, reveal insight into both active pathways and novel regulators of LESCs such as Lrig1 and SOX9 and provide an immense amount of data for future exploration (Bath et al. 2013b). Selective ex vivo expansion of LESCs in hypoxia and the comprehensive molecular characterization of corneal epithelial subpopulations in situ are expected to be beneficial for the future treatment of LSCD by cultured limbal epithelial transplantation.

Transcriptional dissection of human limbal niche compartments by massive parallel sequencing

Corneal epithelium is maintained throughout life by well-orchestrated proliferation of limbal epithelial stem cells (LESCs), followed by migration and maturation centripetally towards the ocular surface. Disturbance of LESCs can potentially lead to a blinding condition, which can be reversed by reconstitution of a functional LESC pool. The current clinical procedures are effective to some degree, however, deeper knowledge of the molecular interplay within the limbal niche is necessary to achieve a fully satisfactory patient outcome. The present study was thus undertaken to carry out a comprehensive transcriptome analysis of four distinct human limbal compartments, including basal limbal crypts (BLCs), superficial limbal crypts (SLCs), cornea, and the supporting stroma, with the aid of laser capture microdissection and deep RNA sequencing. The tissue harvest pipeline was rigorously optimized so that the exposure to cold ischemia would be less than five minutes. The global gene ontology analysis confirmed existence of primitive cells in BLCs, migratory and activated cells in SLCs, and differentiated cells in cornea. Interestingly, many significantly upregulated genes in SLCs mapped to processes involved in regulation of vasculature, such as sFLT1. In contrast, BLCs exhibited many genes mapping to neurogenic processes and processes related to cell development. The primitive nature of BLCs was, furthermore, confirmed by the KEGG pathway analysis, and some potential regulators of LESCs were revealed, such as Lrig1 and SOX9. The analysis also yielded comprehensive lists of uniquely expressed genes in both BLCs and cornea, which may be useful to identify possible biomarkers. In conclusion, the current investigation provides new insight into the relationship between distinct cell populations within the limbal niche, identifies candidates to be verified for novel biological functions, and yields a wealth of information for prospective data mining.