Notch Inhibition Prevents Differentiation of Human Limbal Stem/Progenitor Cells in vitro

Limbal stem cell therapy

PURPOSE OF REVIEW

To highlight the progress and future direction of limbal stem cell (LSC) therapies for the treatment of limbal stem cell deficiency (LSCD).

RECENT FINDINGS

Direct LSC transplantation have demonstrated good long-term outcomes. Cultivated limbal epithelial transplantation (CLET) has been an alternative to treat severe to total LSCD aiming to improve the safety and efficacy of the LSC transplant. A prospective early-stage uncontrolled clinical trial shows the feasibility and safety of CLET manufactured under xenobiotic free conditions. Other cell sources for repopulating of the corneal epithelium such as mesenchymal stem cells (MSCs) and induced pluripotent stem cells are being investigated. The first clinical trials of using MSCs showed short-term results, but long-term efficacy seems to be disappointing. A better understanding of the niche function and regulation of LSC survival and proliferation will lead to the development of medical therapies to rejuvenate the residual LSCs found in a majority of eyes with LSCD in vivo. Prior efforts have been largely focused on improving LSC transplantation. Additional effort should be placed on improving the accuracy of diagnosis and staging of LSCD, and implementing standardized outcome measures which enable comparison of efficacy of different LSCD treatments for different severity of LSCD. The choice of LSCD treatment will be customized based on the severity of LSCD in the future.

SUMMARY

New approaches for managing different stages of LSCD are being developed. This concise review summarizes the progresses in LSC therapies for LSCD, underlying mechanisms, limitations, and future areas of development.

NRG1 Regulates Proliferation, Migration and Differentiation of Human Limbal Epithelial Stem Cells

Limbal epithelial stem/progenitor cells (LESCs) proliferate, migrate and differentiate into mature corneal epithelium cells (CECs) that cover the ocular surface. LESCs play a crucial role in the maintenance and regeneration of the corneal epithelium, and their dysfunction can lead to various corneal diseases. Neuregulin 1 (NRG1) is a member of the epidermal growth factor family that regulates the growth and differentiation of epithelial tissues. Here, we depicted the dynamic transcriptomic profiles during human CEC differentiation, identifying six gene co-expression modules that were specific to different differentiation stages. We found that the expression of NRG1 was high in human LESCs and decreased dramatically upon differentiation. Knockdown of NRG1 significantly inhibited LESC proliferation and upregulated the expression of the terminal differentiation marker genes KRT3, KRT12 and CLU. In addition, the scratch wound closure assay showed that knockdown of NRG1 attenuated wound closure of LESCs over 24 h. Together, we dissected the transcriptional regulatory dynamics during CEC differentiation and identified NRG1 as a key regulator that promoted LESC proliferation and migration and maintained the undifferentiated state.

Extracellular Vesicle MicroRNAs From Corneal Stromal Stem Cell Enhance Stemness of Limbal Epithelial Stem Cells by Targeting the Notch Pathway

Purpose

The limbal niche supports the self-renewal of limbal epithelial stem cells (LESCs). The corneal stromal stem cell (CSSC) is an important component in the niche that regulates the LESC phenotype. However, the intercellular communication between LESCs and CSSCs has yet to be elucidated.

Methods

A traditional two-dimensional (2D) system, a direct three-dimensional (3D) system, and an indirect 3D coculture system of LESCs and CSSCs were used to elucidate the paracrine pathway effect of CSSCs on LESCs. To reveal the impact of CSSC derived extracellular vesicles (CSSC-EVs) on LESCs, GW4869 and CSSC-EVs were added separately to the LESC culture medium. The outgrowth rate, cell density, differentiation, and stemness maintenance were compared among these methods. The miRNAs in the CSSC-EVs were sequenced, and the targeted Notch pathway was further confirmed by RT‒qPCR and Western blotting.

Results

Compared with 2D culture, both the direct and indirect 3D coculture systems yielded a higher outgrowth rate and expression of stem cell markers of LESCs. The phenotypes of LESCs cultivated using the two coculture approaches were also comparable. Nevertheless, GW4869 inhibited the effect of CSSCs on LESCs, and the addition of CSSC-EVs to the 2D culture system could increase cell density, and the proportion of p63αbright cells, which indicated that CSSC-EVs were crucial in regulating LESCs. Furthermore, the EV-AlixKD with reduced miRNA partly lost its regulating function. The abundant miRNAs in CSSC-EVs, such as hsa-miR-663b, hsa-miR-16-5p, and hsa-miR-1290, target the Notch pathway. The LESCs transfected with miR-663b had higher p63 expression via downregulating of the Notch pathway.

Conclusions

CSSC-EV played an important role in promoting LESC proliferation and stemness maintenance by targeting Notch signaling via miRNAs, which will increase our understanding of the limbal niche and provide a potential new approach for LESC culture and the treatment of corneal epithelial disorders.

Therapeutic Strategies for Restoring Perturbed Corneal Epithelial Homeostasis in Limbal Stem Cell Deficiency: Current Trends and Future Directions

Limbal stem cells constitute an important cell population required for regeneration of the corneal epithelium. If insults to limbal stem cells or their niche are sufficiently severe, a disease known as limbal stem cell deficiency occurs. In the absence of functioning limbal stem cells, vision-compromising conjunctivalization of the corneal epithelium occurs, leading to opacification, inflammation, neovascularization, and chronic scarring. Limbal stem cell transplantation is the standard treatment for unilateral cases of limbal stem cell deficiency, but bilateral cases require allogeneic transplantation. Herein we review the current therapeutic utilization of limbal stem cells. We also describe several limbal stem cell markers that impact their phenotype and function and discuss the possibility of modulating limbal stem cells and other sources of stem cells to facilitate the development of novel therapeutic interventions. We finally consider several hurdles for widespread adoption of these proposed methodologies and discuss how they can be overcome to realize vision-restoring interventions.

Hypoxia-induced mesenchymal stem cells inhibit corneal fibroblast proliferation by regulating the WWP2/Notch1 axis

Aim: This study aimed to explore the role of hypoxic mesenchymal stem cells (MSCs) in corneal alkali burns and the underlying mechanism. Materials & methods: Rat corneal fibroblasts were incubated with IL-6, followed by treatment with hypoxic MSC supernatant. A rat corneal alkali burn model was implemented and processed with hypoxic MSCs. The associated factors were detected by corresponding methods. Results: Hypoxic MSCs reduced the Notch1 level and the proliferation of rat corneal fibroblasts. Hypoxic MSCs or WWP2 overexpression in MSCs enhanced ubiquitination of Notch1. WWP2 interacted with Notch1, and WWP2 silencing reversed the effects of the hypoxic MSCs. Hypoxic MSC treatment in vivo decreased the corneal neovascularization scores and opacity scores. Conclusion: Hypoxic MSCs inhibited inflammation and alleviated corneal injury in alkali burns via the WWP2/Notch1 axis.

The c-Myc Oncogene Maintains Corneal Epithelial Architecture at Homeostasis, Modulates p63 Expression, and Enhances Proliferation During Tissue Repair

Purpose

The transcription factor c-Myc (Myc) plays central regulatory roles in both self-renewal and differentiation of progenitors of multiple cell lineages. Here, we address its function in corneal epithelium (CE) maintenance and repair.

Methods

Myc ablation in the limbal–corneal epithelium was achieved by crossing a floxed Myc mouse allele (Myc^fl/fl) with a mouse line expressing the Cre recombinase gene under the keratin (Krt) 14 promoter. CE stratification and protein localization were assessed by histology of paraffin and plastic sections and by immunohistochemistry of frozen sections, respectively. Protein levels and gene expression were determined by western blot and real-time quantitative PCR, respectively. CE wound closure was tracked by fluorescein staining.

Results

At birth, mutant mice appeared indistinguishable from control littermates; however, their rates of postnatal weight gain were 67% lower than those of controls. After weaning, mutants also exhibited spontaneous skin ulcerations, predominantly in the tail and lower lip, and died 45 to 60 days after birth. The mutant CE displayed an increase in stratal thickness, increased levels of Krt12 in superficial cells, and decreased exfoliation rates. Accordingly, the absence of Myc perturbed protein and mRNA levels of genes modulating differentiation and proliferation processes, including ΔNp63β, Ets1, and two Notch target genes, Hey1 and Maml1. Furthermore, Myc promoted CE wound closure and wound-induced hyperproliferation.

Conclusions

Myc regulates the balance among CE stratification, differentiation, and surface exfoliation and promotes the transition to the hyperproliferative state during wound healing. Its effect on this balance may be exerted through the control of multiple regulators of cell fate, including isoforms of tumor protein p63.

Regulation of Limbal Epithelial Stem Cells: Importance of the Niche

Limbal epithelial stem/progenitor cells (LSCs) reside in a niche that contains finely tuned balances of various signaling pathways including Wnt, Notch, BMP, Shh, YAP, and TGFβ. The activation or inhibition of these pathways is frequently dependent on the interactions of LSCs with various niche cell types and extracellular substrates. In addition to receiving molecular signals from growth factors, cytokines, and other soluble molecules, LSCs also respond to their surrounding physical structure via mechanotransduction, interaction with the ECM, and interactions with other cell types. Damage to LSCs or their niche leads to limbal stem cell deficiency (LSCD). The field of LSCD treatment would greatly benefit from an understanding of the molecular regulation of LSCs in vitro and in vivo. This review synthesizes current literature around the niche factors and signaling pathways that influence LSC function. Future development of LSCD therapies should consider all these niche factors to achieve improved long-term restoration of the LSC population.

Human limbal epithelial stem cell regulation, bioengineering and function

The corneal epithelium is continuously renewed by limbal stem/progenitor cells (LSCs), a cell population harbored in a highly regulated niche located at the limbus. Dysfunction and/or loss of LSCs and their niche cause limbal stem cell deficiency (LSCD), a disease that is marked by invasion of conjunctival epithelium into the cornea and results in failure of epithelial wound healing. Corneal opacity, pain, loss of vision, and blindness are the consequences of LSCD. Successful treatment of LSCD depends on accurate diagnosis and staging of the disease and requires restoration of functional LSCs and their niche. This review highlights the major advances in the identification of potential LSC biomarkers and components of the LSC niche, understanding of LSC regulation, methods and regulatory standards in bioengineering of LSCs, and diagnosis and staging of LSCD. Overall, this review presents key points for researchers and clinicians alike to consider in deepening the understanding of LSC biology and improving LSCD therapies.

Wnt6 plays a complex role in maintaining human limbal stem/progenitor cells

The corneal epithelium is consistently regenerated by limbal stem/progenitor cells (LSCs), a very small population of adult stem cells residing in the limbus. Several Wnt ligands, including Wnt6, are preferentially expressed in the limbus. To investigate the role of Wnt6 in regulating proliferation and maintenance of human LSCs in an in vitro LSC expansion setting, we generated NIH-3T3 feeder cells to overexpress different levels of Wnt6. Characterization of LSCs cultured on Wnt6 expressing 3T3 cells showed that high level of Wnt6 increased proliferation of LSCs. Medium and high levels of Wnt6 also increased the percentage of small cells (diameter ≤ 12 µm), a feature of the stem cell population. Additionally, the percentage of cells expressing the differentiation marker K12 was significantly reduced in the presence of medium and high Wnt6 levels. Although Wnt6 is mostly known as a canonical Wnt ligand, our data showed that canonical and non-canonical Wnt signaling pathways were activated in the Wnt6-supplemented LSC cultures, a finding suggesting that interrelationships between both pathways are required for LSC regulation.

A Comparison of Methods for Isolation of Limbal Niche Cells: Maintenance of Limbal Epithelial Stem/Progenitor Cells

Purpose

Limbal niche cells (LNCs) play a vital role in the maintenance of limbal epithelial stem/progenitor cells (LESCs). Four methods have been reported to isolate and expand LNCs: digestion by collagenase alone (C-LNC), collagenase following dispase removal of the limbal epithelium (DC-LNC), dissection of dispase-isolated limbal epithelial sheets (D-LNC), and explant cultures of limbal stromal tissues (Ex-LNC). This study aimed to isolate LNCs using those four methods and to compare their capacity to maintain LESCs.

Methods

LNCs were isolated from the rat corneal limbus by the following methods: C-LNC, DC-LNC, D-LNC, and Ex-LNC. Quantitative real-time PCR and immunofluorescence staining were used to analyze the expression of embryonic stem cell (ESC) markers. The ability to maintain LESCs was assessed on the basis of colony-forming capacity and the expression of progenitor, proliferation, and differentiation markers in three-dimensional (3D) Matrigel and Transwell systems. Notch signaling of LESCs supported by different LNCs in Transwell inserts was analyzed by quantitative real-time PCR.

Results

DC-LNCs exhibited lower expression of CK12 during isolation and expansion. Among P4-expanded LNCs, DC-LNCs expressed significantly higher levels of Sox2, Oct4, Nanog, and N-cadherin than C-LNCs, D-LNCs, and Ex-LNCs. Compared with other LNCs, DC-LNCs were more effective in maintaining LESCs with higher holoclone-forming efficiency, greater expression of ΔNp63α and Ki67, and lower expression of CK12. DC-LNCs were also more capable of downregulating Notch signaling of LESCs.

Conclusions

DC-LNCs were more effective in expressing ESC markers and maintaining LESCs compared to other LNCs. This study identifies an optimal method for the isolation of LNCs in tissue engineering and ocular surface reconstruction.

Inhibition of Notch Signaling Promotes the Differentiation of Epicardial Progenitor Cells into Adipocytes

Background

The role of Notch signaling pathway in the differentiation of epicardial progenitor cells (EPCs) into adipocytes is unclear. The objective is to investigate the effects of Notch signaling on the differentiation of EPCs into adipocytes.

Methods

Frozen sections of C57BL/6J mouse hearts were used to observe epicardial adipose tissue (EAT), and genetic lineage methods were used to trace EPCs. EPCs were cultured in adipogenic induction medium with Notch ligand jagged-1 or γ-secretase inhibitor DAPT. The adipocyte markers, Notch signaling, and adipogenesis transcription factors were determined.

Results

There was EAT located at the atrial-ventricular groove in mouse. By using genetic lineage tracing methods, we found that EPCs were a source of epicardial adipocytes. EPCs had lipid droplet accumulation, and the expression of adipocyte markers FABP-4 and perilipin-1 was upregulated under adipogenic induction. Activating the Notch signaling with jagged-1 attenuated the adipogenic differentiation of EPCs and downregulated the key adipogenesis transcription factor peroxisome proliferator activated receptor-γ (PPAR-γ), while inhibiting the signaling promoted adipogenic differentiation and upregulated PPAR-γ. When blocking PPAR-γ, the role of Notch signaling in promoting adipogenic differentiation was inhibited.

Conclusions

EPCs are a source of epicardial adipocytes. Downregulation of the Notch signaling pathway promotes the differentiation of EPCs into adipocytes via PPAR-γ.

Transcriptional profiles along cell programming into corneal epithelial differentiation

Using the rabbit corneal epithelial cell line RCE1(5T5) as a model, we analyzed three differentiation stages, distinguished on basis to the growth state of cultured cells and after studying the expression of transcription factors such as Oct4, Pax6 and ΔNp63α, selected differentiation markers, and signaling or epigenetic markers such as Notch receptors and Prdm3. Namely, proliferative non-differentiated cells, committed cells, and cells that constitute a stratified epithelium with a limbal epithelial-like structure. RNAseq based transcriptome analysis showed that 4891 genes were differentially expressed among these stages displaying distinctive gene signatures: proliferative cells had 1278 genes as gene signature, and seem to be early epithelial progenitors with an Oct4⁺, KLF4⁺, Myc⁺, ΔNp63α⁺, ABCG2⁺, Vimentin⁺, Zeb1⁺, VANGL1⁺, Krt3^-, Krt12^- phenotype. Committed cells had a gene signature with 417 genes and displayed markers indicative of the beginning of corneal differentiation, and genes characteristic of proliferative cells; we found the possible participation of Six3 and Six4 transcription factors along this stage. The third stage matches with a stratified corneal epithelium (gene signature comprising 979 genes) and is typified by an increase in the expression of WNT10A and NOTCH 2 and 3 signaling and Cux1 transcription factor, besides Pax6, KLF4 or Sox9. The differentiated cells express about 50% of the genes that belong to the Epidermal Differentiation Complex (EDC). Analysis of the differences between corneal epithelium and epidermis could be crucial to understand the regulatory mechanisms that lead to the expression of the differentiated phenotype.

Integrated Transcriptome and Proteome Analyses Reveal the Regulatory Role of miR-146a in Human Limbal Epithelium via Notch Signaling

MiR-146a is upregulated in the stem cell-enriched limbal region vs. central human cornea and can mediate corneal epithelial wound healing. The aim of this study was to identify miR-146a targets in human primary limbal epithelial cells (LECs) using genomic and proteomic analyses. RNA-seq combined with quantitative proteomics based on multiplexed isobaric tandem mass tag labeling was performed in LECs transfected with miR-146a mimic vs. mimic control. Western blot and immunostaining were used to confirm the expression of some targeted genes/proteins. A total of 251 differentially expressed mRNAs and 163 proteins were identified. We found that miR-146a regulates the expression of multiple genes in different pathways, such as the Notch system. In LECs and organ-cultured corneas, miR-146a increased Notch-1 expression possibly by downregulating its inhibitor Numb, but decreased Notch-2. Integrated transcriptome and proteome analyses revealed the regulatory role of miR-146a in several other processes, including anchoring junctions, TNF-α, Hedgehog signaling, adherens junctions, TGF-β, mTORC2, and epidermal growth factor receptor (EGFR) signaling, which mediate wound healing, inflammation, and stem cell maintenance and differentiation. Our results provide insights into the regulatory network of miR-146a and its role in fine-tuning of Notch-1 and Notch-2 expressions in limbal epithelium, which could be a balancing factor in stem cell maintenance and differentiation.

Role of Jagged1-mediated Notch Signaling Activation in the Differentiation and Stratification of the Human Limbal Epithelium

The Notch signaling pathway plays a key role in proliferation and differentiation. We investigated the effect of Jagged 1 (Jag1)-mediated Notch signaling activation in the human limbal stem/progenitor cell (LSC) population and the stratification of the limbal epithelium in vitro. After Notch signaling activation, there was a reduction in the amount of the stem/progenitor cell population, epithelial stratification, and expression of proliferation markers. There was also an increase of the corneal epithelial differentiation. In the presence of Jag1, asymmetric divisions were decreased, and the expression pattern of the polarity protein Par3, normally present at the apical-lateral membrane of basal cells, was dispersed in the cells. We propose a mechanism in which Notch activation by Jag1 decreases p63 expression at the basal layer, which in turn reduces stratification by decreasing the number of asymmetric divisions and increases differentiation.

Top Notch Targeting Strategies in Cancer: A Detailed Overview of Recent Insights and Current Perspectives

Evolutionarily conserved Notch plays a critical role in embryonic development and cellular self-renewal. It has both tumour suppressor and oncogenic activity, the latter of which is widely described. Notch-activating mutations are associated with haematological malignancies and several solid tumours including breast, lung and adenoid cystic carcinoma. Moreover, upregulation of Notch receptors and ligands and aberrant Notch signalling is frequently observed in cancer. It is involved in cancer hallmarks including proliferation, survival, migration, angiogenesis, cancer stem cell renewal, metastasis and drug resistance. It is a key component of cell-to-cell interactions between cancer cells and cells of the tumour microenvironment, such as endothelial cells, immune cells and fibroblasts. Notch displays diverse crosstalk with many other oncogenic signalling pathways, and may drive acquired resistance to targeted therapies as well as resistance to standard chemo/radiation therapy. The past 10 years have seen the emergence of different classes of drugs therapeutically targeting Notch including receptor/ligand antibodies, gamma secretase inhibitors (GSI) and most recently, the development of Notch transcription complex inhibitors. It is an exciting time for Notch research with over 70 cancer clinical trials registered and the first-ever Phase III trial of a Notch GSI, nirogacestat, currently at the recruitment stage.