Maintenance of the keratocyte phenotype during cell proliferation stimulated by insulin

The role of topical insulin in ocular surface restoration: A review

Corneal epithelial defects are one of the most common ocular disorders. Restoring corneal integrity is crucial to reduce pain and regain function, but in cases of neurotrophic or desensitized corneas, healing can be significantly delayed. Treating neurotrophic corneas is challenging for ophthalmologists, and surgical intervention is often indicated to manage refractory cases that are unresponsive to medical therapy. Over the last decade, as more expensive therapeutics reach the market, topical insulin has returned to the forefront as an affordable option to improve corneal wound healing. There is still a paucity of data on the use and the efficacy of topical insulin, with no consensus regarding its indications, preparation, or posology. Here we review the literature on topical insulin for corneal and ocular surface pathologies, with a focus on the current evidence, its mechanisms of action, and its safety profile. Additionally, we share our experience in the field and provide a potential framework for future research.

Molecular cues for immune cells from small leucine-rich repeat proteoglycans in their extracellular matrix-associated and free forms

In this review we highlight emerging immune regulatory functions of lumican, keratocan, fibromodulin, biglycan and decorin, which are members of the small leucine-rich proteoglycans (SLRP) of the extracellular matrix (ECM). These SLRPs have been studied extensively as collagen-fibril regulatory structural components of the skin, cornea, bone and cartilage in homeostasis. However, SLRPs released from a remodeling ECM, or synthesized by activated fibroblasts and immune cells contribute to an ECM-free pool in tissues and circulation, that may have a significant, but poorly understood foot print in inflammation and disease. Their molecular interactions and the signaling networks they influence also require investigations. Here we present studies on the leucine-rich repeat (LRR) motifs of SLRP core proteins, their evolutionary and functional relationships with other LRR pathogen recognition receptors, such as the toll-like receptors (TLRs) to bring some molecular clarity in the immune regulatory functions of SLRPs. We discuss molecular interactions of fragments and intact SLRPs, and how some of these interactions are likely modulated by glycosaminoglycan side chains. We integrate findings on molecular interactions of these SLRPs together with what is known about their presence in circulation and lymph nodes (LN), which are important sites of immune cell regulation. Recent bulk and single cell RNA sequencing studies have identified subsets of stromal reticular cells that express these SLRPs within LNs. An understanding of the cellular source, molecular interactions and signaling consequences will lead to a fundamental understanding of how SLRPs modulate immune responses, and to therapeutic tools based on these SLRPs in the future.

An inspired microenvironment of cell replicas to induce stem cells into keratocyte-like dendritic cells for corneal regeneration

Corneal stromal disorders due to the loss of keratocytes can affect visual impairment and blindness. Corneal cell therapy is a promising therapeutic strategy for healing corneal tissue or even enhancing corneal function upon advanced disorders, however, the sources of corneal keratocytes are limited for clinical applications. Here, the capacity of cell-imprinted substrates fabricated by molding human keratocyte templates to induce differentiation of human adipose-derived stem cells (hADSCs) into keratocytes, is presented. Keratocytes are isolated from human corneal stroma and grown to transmit their ECM architecture and cell-like topographies to a PDMS substrate. The hADSCs are then seeded on cell-imprinted substrates and their differentiation to keratocytes in DMEM/F12 (with and without chemical factors) are evaluated by real-time PCR and immunocytochemistry. The mesenchymal stem cells grown on patterned substrates present gene and protein expression profiles similar to corneal keratocytes. In contrast, a negligible expression of myofibroblast marker in the hADSCs cultivated on the imprinted substrates, is observed. Microscopic analysis reveals dendritic morphology and ellipsoid nuclei similar to primary keratocytes. Overall, it is demonstrated that biomimetic imprinted substrates would be a sufficient driver to solely direct the stem cell fate toward target cells which is a significant achievement toward corneal regeneration.

Insulin facilitates corneal wound healing in the diabetic environment through the RTK-PI3K/Akt/mTOR axis in vitro

Diabetic patients can develop degenerative corneal changes, termed diabetic keratopathy, during the course of their disease. Topical insulin has been shown to reduce corneal wound area and restore sensitivity in diabetic rats, and both the insulin receptor (IR) and insulin-like growth factor 1 receptor (IGF-1R) stimulate cell signaling of the PI3K-Akt pathway. The purpose of this study was to assess a mechanism by which improved wound healing occurs by characterizing expression within the PI3K-Akt pathway in corneal epithelial and stromal cells. In vitro scratch tests were used to evaluate wound healing outcomes under variable glucose conditions in the presence or absence of insulin. Protein expression of intracellular kinases in the PI3K pathway, stromal cell markers, and GLUT-1 was evaluated by immunoblotting.TGF-β1 expression was evaluated by ELISA. Insulin promoted in vitro wound healing in all cell types. In human corneal epithelial cells, insulin did not induce PI3K-Akt signaling; however, in all other cell types evaluated, insulin increased expression of PI3K-Akt signaling proteins compared to vehicle control. Fibroblasts variably expressed α-SMA under all treatment conditions, with significant increases in α-SMA and TGF-β1 occurring in a dose-dependent manner with glucose concentration. These results indicate that insulin can promote corneal cellular migration and proliferation by inducing Akt signaling. Exogenous insulin therapy may serve as a novel target of therapeutic intervention for diabetic keratopathy.

Cell sheet technology: Influence of culture conditions on in vitro-cultivated corneal stromal tissue for regenerative therapies of the ocular surface

The in vitro reconstruction of stromal tissue by long-term cultivation of corneal fibroblasts is a smart approach for regenerative therapies of ocular surface diseases. However, systematic investigations evaluating optimized cultivation protocols for the realization of a biomaterial are lacking. This study investigated the influence of supplements to the culture media of human corneal fibroblasts on the formation of a cell sheet consisting of cells and extracellular matrix. Among the supplements studied are vitamin C, fetal bovine serum, L-glutamine, components of collagen such as L-proline, L-4-hydroxyproline and glycine, and TGF-β1, bFGF, IGF-2, PDGF-BB and insulin. After long-term cultivation, the proliferation, collagen and glycosaminoglycan content and light transmission of the cell sheets were examined. Biomechanical properties were investigated by tensile tests and the ultrastructure was characterized by electron microscopy, small-angle X-ray scattering, antibody staining and ELISA. The synthesis of extracellular matrix was significantly increased by cultivation with insulin or TGF-β1, each with vitamin C. The sheets exhibited a high transparency and suitable material properties. The production of a transparent, scaffold-free, potentially autologous, in vitro-generated construct by culturing fibroblasts with extracellular matrix synthesis-stimulating supplements represents a promising approach for a biomaterial that can be used for ocular surface reconstruction in slowly progressing diseases.

The electrospinning of a thermo-responsive polymer with peptide conjugates for phenotype support and extracellular matrix production of therapeutically relevant mammalian cells

Current cell expansion methods for tissue engineering and regenerative medicine applications rely on the use of enzymatic digestion passaging and 2D platforms. However, this enzymatic treatment significantly reduces cell quality, due to the destruction of important cell-surface proteins. In addition, culture in 2D results in undesired de-differentiation of the cells caused by the lack of 3D similarity to the natural extracellular matrix (ECM) environment. Research has led to the development of thermo-responsive surfaces for the continuous culture of cells. These thermo-responsive materials properties can be used to passage cells from the surface when the cell culture temperature is reduced. Here we report the development of a PLA/thermo-responsive (PDEGMA) blend 3D electrospun fibre-based scaffold to create an enzymatic-free 3D cell culture platform for the expansion of mammalian cells with the desired phenotype for clinical use. Human corneal stromal cells (hCSCs) were used as an exemplar as they have been observed to de-differentiate to an undesirable myo-fibroblastic phenotype when cultured by conventional 2D cell culture methods. Scaffolds were functionalised with a cell adherence peptide sequence GGG-YIGSR by thiol-ene chemistry to improve cell adherence and phenotype support. This was obtained by functionalising the thermo-responsive polymer with a thiol (PDEGMA/PDEGSH) by co-polymerisation. These incorporated thiols react with the norbornene acid functionalised peptide (Nor-GGG-YIGSR) under UV exposure. Presence of the thiol in the scaffold and subsequent peptide attachment on the scaffolds were confirmed by fluorescence labelling, ToF-SIMS and XPS analysis. The biocompatibility of the peptide containing scaffolds was assessed by the adhesion, proliferation and immuno-staining of hCSCs. Significant increase in hCSC adherence and proliferation was observed on the peptide containing scaffolds. Immuno-staining showed maintained expression of the desired phenotypic markers ALDH, CD34 and CD105, while showing no or low expression of the undesired phenotype marker α-SMA. This desired expression was observed to be maintained after thermo-responsive passaging and higher when cells were cultured on PLA scaffolds with 10 wt% PDEGMA/4 mol% PDEGS-Nor-GGG-YIGSR. This paper describes the fabrication and application of a first generation, biocompatible peptide conjugated thermo-responsive fibrous scaffold. The ease of fabrication, successful adherence and expansion of a therapeutically relevant cell type makes these scaffolds a promising new class of materials for the application of cell culture expansion platforms in the biomaterials and tissue engineering field.

Keratocyte biology

The study of corneal stromal keratocytes is motivated by its strong association with corneal health and visual function. They play a dominant role in the maintenance of corneal homeostasis and transparency through the production of collagens, proteoglycans and corneal crystallins. Trauma-induced apoptosis of keratocytes and replacement by fibroblasts and myofibroblasts disrupt the stromal matrix organization, resulting in corneal haze formation and vision loss. It is, therefore, important to understand the biology and behaviours of keratocytes and the associated stromal cell types (like fibroblasts, myofibroblasts, stromal stem cells) in wound healing, corneal pathologies (including keratoconus, keratitis, endothelial disorders) as well as different ophthalmic situations (such as collagen crosslinking/photodynamic treatment, keratoplasty and refractive surgery, and topical medications). The recent development of ex vivo propagation of keratocytes and stromal stem cells, and their translational applications, either via stromal injection or incorporated in bioscaffold, have been shown to restore the corneal transparency and regenerate native stromal tissue in animal models of corneal haze and other disorders.

Influence of micropatterned substrates on keratocyte phenotype

Substrate topographic patterning is a powerful tool that can be used to manipulate cell shape and orientation. To gain a better understanding of the relationship between surface topography and keratocyte behavior, surface patterns consisting of linear aligned or orthogonally aligned microchannels were used. Photolithography and polymer molding techniques were used to fabricate micropatterns on the surface of polydimethylsiloxane (PDMS). Cells on linear aligned substrates were elongated and aligned in the channel direction, while cells on orthogonal substrates had a more spread morphology. Both linear and orthogonal topographies induced chromatin condensation and resulted in higher expressions of keratocyte specific genes and sulfated glycosaminoglycans (sGAG), compared with non-patterned substrates. However, despite differences in cell morphology and focal adhesions, many genes associated with a native keratocyte phenotype, such as keratocan and ALDH3A1, remain unchanged on the different patterned substrates. This information could be used to optimize substrates for keratocyte culture and to develop scaffolds for corneal regeneration.

Fiber reinforced GelMA hydrogel to induce the regeneration of corneal stroma

Regeneration of corneal stroma has always been a challenge due to its sophisticated structure and keratocyte-fibroblast transformation. In this study, we fabricate grid poly (ε-caprolactone)-poly (ethylene glycol) microfibrous scaffold and infuse the scaffold with gelatin methacrylate (GelMA) hydrogel to obtain a 3 D fiber hydrogel construct; the fiber spacing is adjusted to fabricate optimal construct that simulates the stromal structure with properties most similar to the native cornea. The topological structure (3 D fiber hydrogel, 3 D GelMA hydrogel, and 2 D culture dish) and chemical factors (serum, ascorbic acid, insulin, and β-FGF) are examined to study their effects on the differentiation of limbal stromal stem cells to keratocytes or fibroblasts and the phenotype maintenance, in vitro and in vivo tissue regeneration. The results demonstrate that fiber hydrogel and serum-free media synergize to provide an optimal environment for the maintenance of keratocyte phenotype and the regeneration of damaged corneal stroma.

Regeneration of corneal stroma has been a challenge due to its sophisticated structure and the easy transformation of the keratocyte. Here, the authors use a hydrogel reinforced with orthogonally aligned fibres and serum free medium to maintain keratocyte phenotype for the in vivo stromal regeneration.

A thermoresponsive three-dimensional fibrous cell culture platform for enzyme-free expansion of mammalian cells

A three-dimensional thermoresponsive fibrous scaffold system for the subsequent extended culture and enzyme-free passaging of a range of mammalian cell types is presented. Poly(PEGMA₁₈₈) was incorporated with poly(ethylene terephthalate) (PET) via blend-electrospinning to render the fibre thermoresponsive. Using primary human corneal stromal stem cells as an therapeutically relevant exemplar, cell adhesion, viability, proliferation and phenotype on this fibrous culture system over numerous thermal enzyme-free passages is described. We also illustrate the versatility of this system with respect to fabricating thermoresponsive fibres from biodegradable polymers and for the culture of diverse mammalian cell types including mesenchymal stem cells, colon adenocarcinoma cells and NIH-3T3 fibroblasts. This thermoresponsive scaffold system combines the advantages of providing a physiologically relevant environment to maintain a desirable cell phenotype, allowing routine enzyme-free passaging and expansion of cultured cells, whilst offering mechanical support for cell growth. The system described in this study presents a versatile platform for biomedical applications and more specifically for the expansion of mammalian cells destined for the clinic. STATEMENT OF SIGNIFICANCE: The lack of three-dimensional (3D) cell culture environments significantly impacts mammalian cell morphology, proliferation and phenotype in vitro. A versatile, 3D fibrous scaffold system for the extended culture and passaging of a range of clinically-relevant cell types is presented herein. This methodology can be used to fabricate thermoresponsive fibres from polymer blends of any polymer amenable to electrospinning and with a thermoresponsive component. A variety of mammalian cells cultured on the thermoresponsive system were detached from the surface solely by lowering the temperature whilst retaining high viability, a desirable cell phenotype, and supported long-term cell proliferation over numerous thermal enzyme-free passages. This is a significant advance for in vitro expansion of diverse cell types destined for the clinic.

Corneal cell therapy: with iPSCs, it is no more a far-sight

Human-induced pluripotent stem cells (hiPSCs) provide a personalized approach to study conditions and diseases including those of the eye that lack appropriate animal models to facilitate the development of novel therapeutics. Corneal disease is one of the most common causes of blindness. Hence, significant efforts are made to develop novel therapeutic approaches including stem cell-derived strategies to replace the diseased or damaged corneal tissues, thus restoring the vision. The use of adult limbal stem cells in the management of corneal conditions has been clinically successful. However, its limited availability and phenotypic plasticity necessitate the need for alternative stem cell sources to manage corneal conditions. Mesenchymal and embryonic stem cell-based approaches are being explored; nevertheless, their limited differentiation potential and ethical concerns have posed a significant hurdle in its clinical use. hiPSCs have emerged to fill these technical and ethical gaps to render clinical utility. In this review, we discuss and summarize protocols that have been devised so far to direct differentiation of human pluripotent stem cells (hPSCs) to different corneal cell phenotypes. With the summarization, our review intends to facilitate an understanding which would allow developing efficient and robust protocols to obtain specific corneal cell phenotype from hPSCs for corneal disease modeling and for the clinics to treat corneal diseases and injury.

Mechanical stress potentiates the differentiation of periodontal ligament stem cells into keratocytes

Aims

To explore the role of corneal-shaped static mechanical strain on the differentiation of human periodontal ligament stem cells (PDLSCs) into keratocytes and the possible synergistic effects of mechanics and inducing medium.

Methods

PDLSCs were exposed to 3% static dome-shaped mechanical strain in a Flexcell Tension System for 3 days and 7 days. Keratocyte phenotype was determined by gene expression of keratocyte markers. Keratocyte differentiation (inducing) medium was introduced in the Flexcell system, either continuously or intermittently combined with mechanical stimulation. The synergistic effects of mechanics and inducing medium on keratocyte differentiation was evaluated by gene and protein expression of keratocyte markers. Finally, a multilamellar cell sheet was assembled by seeding PDLSCs on a collagen membrane and inducing keratocyte differentiation. The transparency of the cell sheet was assessed, and typical markers of native human corneal stroma were evaluated by immunofluorescence staining.

Results

Dome-shaped mechanical stimulation promoted PDLSCs to differentiate into keratocytes, as shown by the upregulation of ALDH3A1, CD34, LUM, COL I and COL V. The expression of integrins were also upregulated after mechanical stimulation, including integrin alpha 1, alpha 2, beta 1 and non-muscle myosin II B. A synergistic effect of mechanics and inducing medium was found on keratocyte differentiation. The cell sheets were assembled under the treatment of mechanics and inducing medium simultaneously. The cell sheets were transparent, multilamellar and expressed typical markers of corneal stroma.

Conclusion

Dome-shaped mechanical stimulation promotes differentiation of PDLSCs into keratocytes and has synergistic effects with inducing medium. Multilamellar cell sheets that resemble native human corneal stroma show potential for future clinical applications.

The role of hepatocyte growth factor in corneal wound healing

Hepatocyte growth factor (HGF) is a glycoprotein produced by mesenchymal cells and operates as a key molecule for tissue generation and renewal. During corneal injury, HGF is primarily secreted by stromal fibroblasts and promotes epithelial wound healing in a paracrine manner. While this mesenchymal-epithelial interaction is well characterized in various organs and the cornea, the role of HGF in corneal stromal and endothelial wound healing is understudied. In addition, HGF has been shown to play an anti-fibrotic role by inhibiting myofibroblast generation and subsequent production of a disorganized extracellular matrix and tissue fibrosis. Therefore, HGF represents a potential therapeutic tool in numerous organs in which myofibroblasts are responsible for tissue scarring. Corneal fibrosis can be a devastating sequela of injury and can result in corneal opacification and retrocorneal membrane formation leading to severe vision loss. In this article, we concisely review the available literature regarding the role of HGF in corneal wound healing. We highlight the influence of HGF on cellular behaviors in each corneal layer. Additionally, we suggest the possibility that HGF may represent a therapeutic tool for interrupting dysregulated corneal repair processes to improve patient outcomes.

Modeling Keratoconus Using Induced Pluripotent Stem Cells

PURPOSE

To model keratoconus (KC) using induced pluripotent stem cells (iPSC) generated from fibroblasts of both KC and normal human corneal stroma by a viral method.

METHODS

Both normal and KC corneal fibroblasts from four human donors were reprogramed directly by delivering reprogramming factors in a single virus using 2A "self-cleaving" peptides, using a single polycistronic lentiviral vector coexpressing four transcription factors (Oct 4, Sox2, Klf4, and Myc) to yield iPSC. These iPS cells were characterized by immunofluorescence detection using of stem cell markers (SSEA4, Oct4, and Sox2). The mRNA sequencing was performed and the datasets were analyzed using ingenuity pathways analysis (IPA) software.

RESULTS

The generated stem cell-like clones expressed the pluripotency markers, SSEA4, Oct4, Sox2, Tra-1-60, and also expressed pax6. Our transcriptome analysis showed 4300 genes, which had 2-fold change and 870 genes with a q-value of <0.05 in keratoconus iPSC compared to normal iPSC. One of the genes that showed difference in KC iPSC was FGFR2 (down-regulated by 2.4 fold), an upstream target of Pi3-Kinase pathway, was further validated in keratoconus corneal sections and also KC iPSC-derived keratocytes (down regulated by 2.0-fold). Both normal and KC-derived keratocytes expressed keratocan, signature marker for keratocytes. KC iPSC-derived keratocytes showed adverse growth and proliferation and was further confirmed by using Ly2924002, a PI3k inhibitor, which severely affected the growth and differentiation in normal iPSC.

CONCLUSIONS

Based on our result, we propose a model for KC in which inhibition FGFR2-Pi3-Kinase pathway affects the AKT phosphorylation, and thus affecting the keratocytes survival signals. This inhibition of the survival signals could be a potential mechanism for the KC-specific decreased cell survival and apoptosis of keratocytes.

Tissue-engineered cornea constructed with compressed collagen and laser-perforated electrospun mat

While Plastic Compressed (PC) collagen technique is often used to fabricate bioengineered constructs, PC collagen gels are too weak to be sutured or conveniently handled for clinical applications. To overcome this limitation, electrospun poly (lactic-co-glycolide) (PLGA) mats, which have excellent biocompatibility and mechanical properties, were combined with PC collagen to fabricate sandwich-like hybrid constructs. By laser-perforating holes with different sizes and spacings in the electrospun mats to regulate the mechanical properties and light transmittance of the hybrid constructs, we produced hybrid constructs with properties very suitable to apply in corneal tissue engineering. The maximum tensile stress of the optimal hybrid construct was 3.42 ± 0.22 MPa. The light transmittance of the hybrid construct after perforation was approximately 15-fold higher than before, and light transmittance increased gradually with increasing time. After immersing into PBS for 7 days, the transmittance of the optimal construct changed from 63 ± 2.17% to 72 ± 1.8% under 500 nm wavelength. The live/dead staining, cell proliferation assay and immunohistochemistry study of human corneal epithelial cells (HCECs) and human keratocytes (HKs) cultured on the optimal hybrid construct both demonstrated that the cells adhered, proliferated, and maintained their phenotype well on the material. In addition, after culturing for 2 weeks, the HCECs could form stratified layers. Thus, our designed construct is suitable for the construction of engineered corneal tissue.

Corneal keratocyte transition to mesenchymal stem cell phenotype and reversal using serum-free medium supplemented with fibroblast growth factor-2, transforming growth factor-β3 and retinoic acid

Keratocytes of the corneal limbal stroma can derive populations of mesenchymal stem cells (MSC) when expanded in vitro. However, once a corneal MSC (cMSC) phenotype is achieved, regaining the keratocyte phenotype can be challenging, and there is no standardised differentiation medium. Here, we investigated the transition of keratocytes to cMSC and compared different supplements in their ability to return cMSC to a keratocyte phenotype. Immunofluorescence and quantitative reverse transcription polymerase chain reaction demonstrated in vivo keratocyte expression of aldehyde dehydrogenase 3A1, CD34 and keratocan, but not any of the typical MSC markers (CD73, CD90, CD105). As the keratocytes were expanded in vitro, the phenotypic profile reversed and the cells expressed MSC markers but not keratocyte markers. Differentiating the cMSC back to a keratocyte phenotype using nonsupplemented, serum-free medium restored keratocyte markers but did not maintain cell viability or support corneal extracellular matrix production. Supplementing the differentiation medium with combinations of fibroblast growth factor-2, transforming growth factor-β3 and retinoic acid maintained viability, restored expression of CD34, aldehyde dehydrogenase 3A1 and keratocan, and facilitated production of abundant extracellular matrix as shown by immunofluorescent staining for collagen-I and lumican, alongside quantitative assays for collagen and glycosaminoglycan production. However, no differentiation medium was able to downregulate the expression of MSC markers in the 21-day culture period. This study shows that the keratocyte to MSC transition can be partially reversed using serum-free media and supplementation with retinoic acid, fibroblast growth factor-2 and transforming growth factor-β3 and can enhance this effect. This is relevant for development of corneal regenerative strategies that require the production of a keratocyte phenotype. Copyright © 2016 John Wiley & Sons, Ltd.

Low oxygen tension and macromolecular crowding accelerate extracellular matrix deposition in human corneal fibroblast culture

Development of implantable devices based on the principles of in vitro organogenesis has been hindered due to the prolonged time required to develop an implantable device. Herein we assessed the influence of serum concentration (0.5% and 10%), oxygen tension (0.5%, 2% and 20%) and macromolecular crowding (75 μg/ml carrageenan) in extracellular matrix deposition in human corneal fibroblast culture (3, 7 and 14 days). The highest extracellular matrix deposition was observed after 14 days in culture at 0.5% serum, 2% oxygen tension and 75 μg/ml carrageenan. These data indicate that low oxygen tension coupled with macromolecular crowding significantly accelerate the development of scaffold-free tissue-like modules. Copyright © 2016 John Wiley & Sons, Ltd.

Retinoic Acid Enhances the Differentiation of Adipose-Derived Stem Cells to Keratocytes In Vitro

Purpose

All-trans retinoic acid (RA) supplementation was investigated as a method of enhancing the differentiation of human adipose-derived stem cells (ASCs) to corneal keratocytes in vitro, in combination with a chemically defined serum-free medium.

Methods

Adipose-derived stem cells were cultured in monolayer and supplemented with 0.1, 1, or 10 μM RA for 14 days. The effects of RA on cell proliferation, migration, and extracellular matrix (ECM) accumulation were evaluated. In addition, the expression of phenotypic keratocyte markers was examined by reverse transcription polymerase chain reaction (PCR), immunocytochemistry, and Western blotting.

Results

Adipose-derived stem cells cultured with RA showed improved cell proliferation and ECM production. In addition, RA enhanced the expression of keratocyte-specific markers, keratocan, aldehyde dehydrogenase 3A1, lumican, and decorin, when compared to serum-free media alone. Furthermore, the presence of RA increased the amount of collagen type I while reducing the expression of fibrotic marker, α-smooth muscle actin.

Conclusions

These findings indicate that RA is a useful supplement for promoting a keratocyte phenotype in ASC.

Translational Relevance

This study is particularly important for the generation of biological corneal substitutes and next generation cell based therapies for corneal conditions.

Effects of insulin on the skin: possible healing benefits for diabetic foot ulcers

Diabetic foot ulcers affect 15-20 % of all diabetic patients and remain an important challenge since the available therapies have limited efficacy and some of the novel therapeutic approaches, which include growth factors and stem cells, are highly expensive and their safety remains to be evaluated. Despite its low cost and safety, the interest for topical insulin as a healing agent has increased only in the last 20 years. The molecular mechanisms of insulin signaling and its metabolic effects have been well studied in its classical target tissues. However, little is known about the specific effects of insulin in healthy or even diabetic skin. In addition, the mechanisms involved in the effects of insulin on wound healing have been virtually unknown until about 10 years ago. This paper will review the most recent advances in the cellular and molecular mechanisms that underlie the beneficial effects of insulin on skin wound healing in diabetes. Emerging evidence that links dysfunction of key cellular organelles, namely the endoplasmic reticulum and the mitochondria, to changes in the autophagy response, as well as the impaired wound healing in diabetic patients will also be discussed along with the putative mechanisms whereby insulin could regulate/modulate these alterations.

The effect of growth factor supplementation on corneal stromal cell phenotype in vitro using a serum-free media

In order to expand cells quickly and in high numbers for corneal tissue engineering applications corneal stromal cells, or keratocytes, are often cultured in the presence of serum. However, keratocytes become fibroblastic when exposed to serum leading to a downregulation of corneal stromal specific markers. The purpose of this current study was to determine if corneal stromal cells, made fibroblastic by serum, could display native quiescent keratocyte characteristics when cultured under serum-free conditions supplemented by different growth factors. Markers specific to a native keratocyte phenotype such as keratocan and aldehyde dehydrogenase 3A1 (ALDH3A1) and those specific to a fibrotic phenotype such as α-smooth muscle actin (αSMA) and collagen type III were examined. Cells were cultured in monolayer, self-assembled pellets or collagen hydrogels. Growth factors known to modulate keratocyte phenotype were chosen to supplement the serum free media, specifically insulin-like growth factor 1 (IGF-1) and transforming growth factor beta 1 and 3 (Tβ1 and Tβ3). The effects of serum-free media, growth factors and culture system on cell proliferation and morphology and extracellular matrix (ECM) synthesis were evaluated. The expression of keratocyte markers was evaluated by real-time PCR, immunofluorescent staining and western blotting. In addition, cell migration was tested using scratch assays. When serum was removed from the cells they displayed a reduction in proliferation and ECM synthesis (not significant), in addition to a significant decrease in migratory capacity (p < 0.05). Serum-free media promoted increased expression of keratocan (130.68 ± 47.44-fold increase; p < 0.05) and collagen type I (15.58 ± 9.49-fold increase; p < 0.05). However, there was no significant change in ALDH3A1 and αSMA expression, while collagen type III expression was significantly increased (44.66 ± 25.61-fold increase; p < 0.05). In addition, cells retained an elongated fibroblastic morphology. In monolayer, the addition of Tβ1 and Tβ3 to serum free media resulted in reduced expression of keratocan, ALDH3A1 and collagen type I and III, increased expression of αSMA (p < 0.05) and an increase in cell proliferation and ECM synthesis. Pellet cultured cells demonstrated a significant increase in ALDH3A1 and collagen type I over 14 days relative to day 5 (p < 0.05), however the expression of fibrotic markers was also enhanced. Cells in collagen hydrogels did not increase expression of keratocyte markers in serum free conditions and underwent contraction in Tβ1 and Tβ3 supplemented media. These results demonstrate that corneal fibroblasts only partially express the phenotypic characteristics of keratocytes when cultured in serum-free medium. While growth factors did not significantly enhance this phenotype, it appears that pellet or self-assembled culture could be more beneficial to promoting a keratocyte phenotype.

Progress in Corneal Stromal Repair: From Tissue Grafts and Biomaterials to Modular Supramolecular Tissue-Like Assemblies

Corneal injuries and degenerative conditions have major socioeconomic consequences, given that in most cases, they result in blindness. In the quest of the ideal therapy, tissue grafts, biomaterials, and modular engineering approaches are under intense investigation. Herein, advancements and shortfalls are reviewed and future perspectives for these therapeutic strategies discussed.

Substance P Enhances Keratocyte Migration and Neutrophil Recruitment through Interleukin-8

Keratocytes, the resident cells of the corneal stroma, are responsible for maintaining turnover of this tissue by synthesizing extracellular matrix components. When the cornea is injured, the keratocytes migrate to the wounded site and participate in the stromal wound healing. The neuropeptide substance P (SP), which is also known to be produced by non-neuronal cells, has previously been implicated in epithelial wound healing after corneal injury. Corneal scarring, which occurs in the stroma when the process of wound healing has malfunctioned, is one of the major causes of preventable blindness. This study aimed to elucidate the potential role of SP in keratocyte migration and therefore in stromal wound healing. We report that the expression and secretion of SP in human keratocytes are increased in response to injury in vitro. Moreover, SP enhances the migration of keratocytes by inducing the actin cytoskeleton reorganization and focal adhesion formation through the activation of the phosphatidylinositide 3-kinase and Ras-related C3 botulinum toxin substrate 1/Ras homolog gene family, member A pathway. Furthermore, SP stimulation leads to upregulated expression of the proinflammatory and chemotactic cytokine interleukin-8 (IL-8), which also contributes significantly to SP-enhanced keratocyte migration and is able to attract neutrophils. In addition, the preferred SP receptor, the neurokinin-1 receptor, is necessary to induce keratocyte migration and IL-8 secretion. In conclusion, we describe new mechanisms by which SP enhances migration of keratocytes and recruits neutrophils, two necessary steps in the corneal wound-healing process, which are also likely to occur in other tissue injuries.

Expression Profiles of Neuropeptides, Neurotransmitters, and Their Receptors in Human Keratocytes In Vitro and In Situ

Keratocytes, the quiescent cells of the corneal stroma, play a crucial role in corneal wound healing. Neuropeptides and neurotransmitters are usually associated with neuronal signaling, but have recently been shown to be produced also by non-neuronal cells and to be involved in many cellular processes. The aim of this study was to assess the endogenous intracellular and secreted levels of the neuropeptides substance P (SP) and neurokinin A (NKA), and of the neurotransmitters acetylcholine (ACh), catecholamines (adrenaline, noradrenaline and dopamine), and glutamate, as well as the expression profiles of their receptors, in human primary keratocytes in vitro and in keratocytes of human corneal tissue sections in situ. Cultured keratocytes expressed genes encoding for SP and NKA, and for catecholamine and glutamate synthesizing enzymes, as well as genes for neuropeptide, adrenergic and ACh (muscarinic) receptors. Keratocytes in culture produced SP, NKA, catecholamines, ACh, and glutamate, and expressed neurokinin-1 and -2 receptors (NK-1R and NK-2R), dopamine receptor D₂, muscarinic ACh receptors, and NDMAR1 glutamate receptor. Human corneal sections expressed SP, NKA, NK-1R, NK-2R, receptor D₂, choline acetyl transferase (ChAT), M₃, M₄ and M₅ muscarinic ACh receptors, glutamate, and NMDAR1, but not catecholamine synthesizing enzyme or the α₁ and β₂ adrenoreceptors, nor M₁ receptor. In addition, expression profiles assumed significant differences between keratocytes from the peripheral cornea as compared to those from the central cornea, as well as differences between keratocytes cultured under various serum concentrations. In conclusion, human keratocytes express an array of neuropeptides and neurotransmitters. The cells furthermore express receptors for neuropeptides/neurotransmitters, which suggests that they are susceptible to stimulation by these substances in the cornea, whether of neuronal or non-neuronal origin. As it has been shown that neuropeptides/neurotransmitters are involved in cell proliferation, migration, and angiogenesis, it is possible that they play a role in corneal wound healing.

Low-glucose enhances keratocyte-characteristic phenotype from corneal stromal cells in serum-free conditions

The avascular cornea is a uniquely-isolated organ, with its stroma constituting a nutrient-poor environment. Consequently, the availability of metabolites such as glucose to corneal stromal cells is considerably reduced compared with other tissues, or indeed with media commonly used to culture these cells in vitro. However, the role of glucose in the behaviour of human corneal keratocytes has been overlooked. As such, we sought to investigate the effects of low-glucose formulations on the phenotype of human corneal stromal cells. Cells cultured in low-glucose were able to survive for extended periods when compared to high-glucose, serum-free conditions. Furthermore, low-glucose enhanced their reversal to a keratocyte-characteristic phenotype. Specifically, cells within low-glucose medium assumed dendritic morphologies, with bean-shaped condensed nuclei, absence of alpha-smooth muscle actin or stress fibres, and a corresponding reduction in migratory and contractile activities when compared with high-glucose, serum-free conditions. Moreover, cells within low-glucose uniquely recovered the ability to express a robust keratocyte-characteristic marker, CD34, while still expressing elevated levels of other representative phenotypic markers such as keratocan, lumican, ALDH1A1, and ALDH3A1. These results indicate that low-glucose enhances keratocyte-characteristic phenotype above and beyond established media formulations and thus has important implications for corneal biology in health and disease.

Accelerated Development of Supramolecular Corneal Stromal-Like Assemblies from Corneal Fibroblasts in the Presence of Macromolecular Crowders

Tissue engineering by self-assembly uses the cells' secretome as a regeneration template and biological factory of trophic factors. Despite the several advantages that have been witnessed in preclinical and clinical settings, the major obstacle for wide acceptance of this technology remains the tardy extracellular matrix formation. In this study, we assessed the influence of macromolecular crowding (MMC)/excluding volume effect, a biophysical phenomenon that accelerates thermodynamic activities and biological processes by several orders of magnitude, in human corneal fibroblast (HCF) culture. Our data indicate that the addition of negatively charged galactose derivative (carrageenan) in HCF culture, even at 0.5% serum, increases by 12-fold tissue-specific matrix deposition, while maintaining physiological cell morphology and protein/gene expression. Gene analysis indicates that a glucose derivative (dextran sulfate) may drive corneal fibroblasts toward a myofibroblast lineage. Collectively, these results indicate that MMC may be suitable not only for clinical translation and commercialization of tissue engineering by self-assembly therapies, but also for the development of in vitro pathophysiology models.

Macromolecularly crowded in vitro microenvironments accelerate the production of extracellular matrix-rich supramolecular assemblies

Therapeutic strategies based on the principles of tissue engineering by self-assembly put forward the notion that functional regeneration can be achieved by utilising the inherent capacity of cells to create highly sophisticated supramolecular assemblies. However, in dilute ex vivo microenvironments, prolonged culture time is required to develop an extracellular matrix-rich implantable device. Herein, we assessed the influence of macromolecular crowding, a biophysical phenomenon that regulates intra- and extra-cellular activities in multicellular organisms, in human corneal fibroblast culture. In the presence of macromolecules, abundant extracellular matrix deposition was evidenced as fast as 48 h in culture, even at low serum concentration. Temperature responsive copolymers allowed the detachment of dense and cohesive supramolecularly assembled living substitutes within 6 days in culture. Morphological, histological, gene and protein analysis assays demonstrated maintenance of tissue-specific function. Macromolecular crowding opens new avenues for a more rational design in engineering of clinically relevant tissue modules in vitro.

Ex Vivo Propagation of Human Corneal Stromal "Activated Keratocytes" for Tissue Engineering

Keratoconus is a corneal disorder characterized by a thinning of stromal tissue, and the affected patients have induced astigmatism and visual impairment. It is associated with a loss of corneal stromal keratocytes (CSKs). Hence, reconstructing stromal tissue with autologous CSK replacement can be a viable alternative to corneal transplantation, which is restricted by the global donor material shortage and graft rejection. Human CSKs are normally quiescent and express unique markers, like aldehyde dehydrogenases and keratocan. In serum culture, they proliferate, but lose their characteristic phenotype and become stromal fibroblasts. Here we report a novel culture cocktail to ex vivo propagate and maintain CSKs. Primary human CSKs were obtained from adult donors and cultured with soluble human amnion stromal extract (ASE), rho-associated coiled-coil-forming protein serine/threonine kinase inhibitor Y-27632, and insulin-like growth factor-1 (collectively named as ERI). Protein profiling using mass spectrometry followed by MetaCore™ pathway analysis predicted that ASE proteins might participate in transforming growth factor-β (TGF-β) signaling and fibroblast development, cell adhesion, extracellular matrix remodeling, and immune response. In culture with 0.5% fetal bovine serum and ERI, the population of "activated keratocytes" was expanded. They had much lowered expression of both keratocyte and fibroblast markers, suppressed TGF-β-mediated Smad2/3 activation, and lacked fibroblast-mediated collagen contractibility. These "activated keratoctyes" could be propagated for six to eight passages ex vivo, and they regained CSK-specific dendritic morphology and gene marker expression, including aldehyde dehydrogenases, lumican, and keratocan biosynthesis, expression, and secretion when returned to serum-depleted ERI condition. This novel cocktail maintained human CSKs in both adherent and suspension cultures with proper keratocyte features and without the transformation to stromal fibroblasts. Thus, human CSKs can be ex vivo propagated as transient "activated keratocytes." This could provide sufficient number of genuine CSKs for corneal tissue engineering.

Insulin and wound healing

Skin is a dynamic and complex organ that relies on the interaction of different cell types, biomacromolecules and signaling molecules. Injury triggers a cascade of events designed to quickly restore skin integrity. Depending on the size and severity of the wound, extensive physiological and metabolic changes can occur, resulting in impaired wound healing and increased morbidity resulting in higher rates of death. While wound dressings provide a temporary barrier, they are inherently incapable of significantly restoring metabolic upsets, post-burn insulin resistance, and impaired wound healing in patients with extensive burns. Exogenous insulin application has therefore been investigated as a potential therapeutic intervention for nearly a century to improve wound recovery. This review will highlight the important achievements that demonstrate insulin's ability to stimulate cellular migration and burn wound recovery, as well as providing a perspective on future therapeutic applications and research directions.

A role for topographic cues in the organization of collagenous matrix by corneal fibroblasts and stem cells

Human corneal fibroblasts (HCF) and corneal stromal stem cells (CSSC) each secrete and organize a thick stroma-like extracellular matrix in response to different substrata, but neither cell type organizes matrix on tissue-culture polystyrene. This study compared cell differentiation and extracellular matrix secreted by these two cell types when they were cultured on identical substrata, polycarbonate Transwell filters. After 4 weeks in culture, both cell types upregulated expression of genes marking differentiated keratocytes (KERA, CHST6, AQP1, B3GNT7). Absolute expression levels of these genes and secretion of keratan sulfate proteoglycans were significantly greater in CSSC than HCF. Both cultures produced extensive extracellular matrix of aligned collagen fibrils types I and V, exhibiting cornea-like lamellar structure. Unlike HCF, CSSC produced little matrix in the presence of serum. Construct thickness and collagen organization was enhanced by TGF-ß3. Scanning electron microscopic examination of the polycarbonate membrane revealed shallow parallel grooves with spacing of 200–300 nm, similar to the topography of aligned nanofiber substratum which we previously showed to induce matrix organization by CSSC. These results demonstrate that both corneal fibroblasts and stromal stem cells respond to a specific pattern of topographical cues by secreting highly organized extracellular matrix typical of corneal stroma. The data also suggest that the potential for matrix secretion and organization may not be directly related to the expression of molecular markers used to identify differentiated keratocytes.

Corneal stromal cell plasticity: in vitro regulation of cell phenotype through cell-cell interactions in a three-dimensional model

In vivo, epithelial cells are connected both anatomically and functionally with stromal keratocytes. Co-culturing aims at recapturing this cellular anatomy and functionality by bringing together two or more cell types within the same culture environment. Corneal stromal cells were activated to their injury phenotype (fibroblasts) and expanded before being encapsulated in type I collagen hydrogels constructs. Three different epithelial-stromal co-culture methods were then examined: epithelial explant; transwell; and the use of conditioned media. The aim was to determine whether the native, inactivated keratocyte cell phenotype could be restored in vitro. Media supplementation with transforming growth factor beta-1 (TGF-β1) was then used to determine whether the inactivated stromal cells retained their plasticity in vitro and could be re-activated to the fibroblast phenotype. Finally, media supplementation with wortmannin was used to inhibit epithelial-stromal cell interactions. Two different nondestructive techniques, spherical indentation and optical coherence tomography, were used to reveal how epithelial-stromal co-culturing with TGF-β1, and wortmannin media supplementation, respectively, affect stromal cell behavior and differentiation in terms of construct contraction and elastic modulus measurement. Cell viability, phenotype, morphology, and protein expression were investigated to corroborate our mechanical findings. It was shown that activated stromal cells could be inactivated to a keratocyte phenotype via co-culturing and that they retained their plasticity in vitro. Activated corneal stromal cells that were fibroblastic in phenotype were successfully reverted to a nonactivated keratocyte cell lineage in terms of behavior and biological properties; and then back again via TGF-β1 media supplementation. It was then revealed that epithelial-stromal interactions can be blocked via the use of wortmannin inhibition. A greater understanding of stromal-epithelial interactions and what mediates them offers great pharmacological potential in the regulation of corneal wound healing, with the potential to treat corneal diseases and injury by which such interactions are vital.

Characterization of corneal stromal stem cells with the potential for epithelial transdifferentiation

INTRODUCTION

The corneal stroma is being increasingly recognized as a repository for stem cells. Like the limbal and endothelial niches, stromal stem cells often reside in the peripheral cornea and limbus. These peripheral and limbal corneal stromal cells (PLCSCs) are known to produce mesenchymal stem cells in vitro. Recently, a common corneal stromal and epithelial progenitor was hinted at. This study aims to examine the stem cell potential of corneal stromal cells and to investigate their epithelial transdifferentiation ability.

METHODS

PLCSCs were grown in traditional Dulbecco modified Eagle medium (DMEM)-based keratocyte culture medium and an M199-based medium and analyzed for a profile of cell-surface markers by using flow cytometry and differentiated into mesenchymal phenotypes analyzed with quantitative polymerase chain reaction (qPCR) and histologic staining. PLCSCs in M199 were subsequently divided into subpopulations based on CD34 and CD105 expression by using fluorescence- activated cell sorting (FACS). Subpopulations were characterized by marker profile and mesenchymal differentiation ability. Both whole PLCSCs and subpopulations were also cultured for epithelial transdifferentiation.

RESULTS

Cells cultured in M199 demonstrated a more stem-like cell-surface marker profile, and the keratocyte marker CD34 was retained for several passages but absent in cells cultured in DMEM. Cells cultured in M199 also exhibited a greater mesenchymal differentiation potential, compared with DMEM. PLCSCs could be divided into CD34(+)CD105(+), CD34-CD105(+), and CD34-CD105- subpopulations, of which CD34(+)CD105(+) cells were the most stemlike with regard to marker expression and mesenchymal differentiation potential. Subpopulations of PLCSCs exhibited differing abilities to transdifferentiate into epithelial phenotypes. Cells that were initially CD34(+)CD105(+) showed the greatest differentiation potential, producing CK3(+) and CK19(+) cells, and expressed a range of both epithelial progenitor (HES1, FRZB1, DCT, SOD2, ABCG2, CDH1, KRT19) and terminally differentiated (DSG3, KRT3, KRT12, KRT24) genes.

CONCLUSIONS

Culture medium has a significant effect on the phenotype and differentiation capacity of PLCSCs. The stroma contains a heterogeneous cell population in which we have identified CD34(+) cells as a stem cell population with a capacity for mesenchymal and epithelial differentiation.

Three-dimensional cell culture environment promotes partial recovery of the native corneal keratocyte phenotype from a subcultured population

Corneal disease is the fourth leading cause of blindness. According to the World Health Organization, roughly 1.6 million people globally are blind as a result of this disease. The only current treatment for corneal opacity is a corneal tissue transplant. Unfortunately, the demand for tissue exceeds supply, making a tissue-engineered in vitro cornea highly desirable. For an in vitro cornea to be useful, it must be transparent, which requires downregulation of the light-scattering intracellular protein alpha-smooth muscle actin (αSMA) and upregulation of the native corneal marker, aldehyde dehydrogenase 1A1 (ALDH1A1). This study focuses on the effects of a three-dimensional (3D) matrix on the expression levels of αSMA and ALDH1A1 by a subcultured population of rabbit corneal keratocytes and the comparison of the 3D matrix effects to other culture conditions. We show that, through western blot and quantitative real-time PCR, the presence of collagen strongly downregulates αSMA. Further, 3D cultures maintain low actin expression even in the presence of a proinflammatory cytokine, transforming growth factor-beta (TGF-β). Finally, 3D culture conditions show a partial recovery of ALDH1A1 expression, which has never been previously observed in a serum-exposed subcultured cell population. Overall, this study suggests that 3D culture is not only a relatively stronger signal than both collagen and TGF-β, it is also sufficient to induce some recovery of ALDH1A1 and the native corneal phenotype despite the presence of serum.

Sphere formation from corneal keratocytes and phenotype specific markers

The keratocytes are specialized mesenchymal cells that produce and maintain the extracellular matrix of the corneal stroma. With a typical dendritic and flattened appearance, these cells can morph into fibroblasts and myofibroblasts upon injury, and produce abnormal or fibrotic extracellular matrices detrimental to corneal transparency. Insights into mechanisms that regulate these phenotypic switches and optimal culture conditions that preserve the keratocyte phenotype are important for tissue engineering of the corneal stroma. Like other cell types with self-renewing capacity, keratocytes can form spheres in culture. Here we investigated human and bovine keratocytes with respect to their sphere forming capabilities, and sought to identify potentially distinguishing markers for the keratocyte and fibroblast phenotypes. Keratocytes, isolated from bovine and human corneas, cultured in serum-free medium supplemented with insulin, selenium and transferrin, assumed typical keratocyte morphology, converted to fibroblasts in serum-containing medium and reverted to keratocytes after serum-deprivation. The bovine keratocytes produced spheres under adherent or low attachment conditions, while the human keratocytes produced spheres under low attachment conditions only. The primary keratocytes and fibroblasts expressed vimentin, confirming their mesenchymal origin. Keratocan, considered to be a marker for keratocytes, was also detected in early passage bovine fibroblasts. BMP3 was expressed in keratocytes and keratocyte-derived spheres, while cadherin 5 in keratocytes only, suggesting these as potential keratocyte markers.

Transforming growth factor-β3 regulates assembly of a non-fibrotic matrix in a 3D corneal model

Corneal tissue engineering has attracted the attention of many researchers over the years, in part due to the cornea's avascularity and relatively straightforward structure. However, the highly organized and structured nature of this optically clear tissue has presented a great challenge. We have previously developed a model in which human corneal fibroblasts (HCFs) are stimulated by a stable vitamin C (VitC) derivative to self-assemble an extracellular matrix (ECM). Addition of TGFβ1 enhanced the assembly of ECM; however, it was accompanied by the upregulation of specific fibrotic markers. In this study, we tested the effects of all three TGFβ isoforms (-β1, -β2 and -β3) on ECM production, as well as expression of fibrotic markers. HCFs were grown in four media conditions for 4 weeks: control, VitC only; T1, VitC + TGFβ1; T2, VitC + TGFβ2; and T3, VitC + TGFβ3. The cultures were analysed with western blots, TEM and indirect immunofluorescence (IF). Compared to controls, all TGFβ isoforms stimulated matrix production by about three-fold. IF showed the presence of type III collagen and smooth muscle actin (SMA) in T1 and T2; however, T3 showed little to no expression. In western blots, T3 stimulated a lower type III:type I collagen ratio when compared to the other conditions. In addition, TEM indicated that T3 stimulated a higher level of matrix alignment and organization. HCFs stimulated by VitC and TGFβ3 appear to generate a matrix that mimics the normal adult or developing human cornea, whereas TGF-β1 and -β2 drive the constructs towards a more fibrotic path.

The molecular basis of corneal transparency

The cornea consists primarily of three layers: an outer layer containing an epithelium, a middle stromal layer consisting of a collagen-rich extracellular matrix (ECM) interspersed with keratocytes and an inner layer of endothelial cells. The stroma consists of dense, regularly packed collagen fibrils arranged as orthogonal layers or lamellae. The corneal stroma is unique in having a homogeneous distribution of small diameter 25-30 nm fibrils that are regularly packed within lamellae and this arrangement minimizes light scattering permitting transparency. The ECM of the corneal stroma consists primarily of collagen type I with lesser amounts of collagen type V and four proteoglycans: three with keratan sulfate chains; lumican, keratocan, osteoglycin and one with a chondroitin sulfate chain; decorin. It is the core proteins of these proteoglycans and collagen type V that regulate the growth of collagen fibrils. The overall size of the proteoglycans are small enough to fit in the spaces between the collagen fibrils and regulate their spacing. The stroma is formed during development by neural crest cells that migrate into the space between the corneal epithelium and corneal endothelium and become keratoblasts. The keratoblasts proliferate and synthesize high levels of hyaluronan to form an embryonic corneal stroma ECM. The keratoblasts differentiate into keratocytes which synthesize high levels of collagens and keratan sulfate proteoglycans that replace the hyaluronan/water-rich ECM with the densely packed collagen fibril-type ECM seen in transparent adult corneas. When an incisional wound through the epithelium into stroma occurs the keratocytes become hypercellular myofibroblasts. These can later become wound fibroblasts, which provides continued transparency or become myofibroblasts that produce a disorganized ECM resulting in corneal opacity. The growth factors IGF-I/II are likely responsible for the formation of the well organized ECM associated with transparency produced by keratocytes during development and by the wound fibroblast during repair. In contrast, TGF-beta would cause the formation of the myofibroblast that produces corneal scaring. Thus, the growth factor mediated synthesis of several different collagen types and the core proteins of several different leucine-rich type proteoglycans as well as posttranslational modifications of the collagens and the proteoglycans are required to produce collagen fibrils with the size and spacing needed for corneal stromal transparency.

Meet the corneal myofibroblast: the role of myofibroblast transformation in corneal wound healing and pathology

In recent years the role of the myofibroblast in corneal wound healing has been extensively studied. The transformation of the corneal keratocyte to the fibroblast and myofibroblast phenotypes is a critical process in normal and pathologic corneal wound healing. This review will introduce this important transformation and its role in corneal wound healing and pathology.

Effect of serum and insulin modulation on the organization and morphology of matrix synthesized by bovine corneal stromal cells

The in vitro production of highly organized collagen fibrils by corneal keratocytes in a three-dimensional scaffold-free culture system presents a unique opportunity for the direct observation of organized matrix formation. The objective of this investigation was to develop such a culture system in a glass substrate (for optical accessibility) and to directly examine the effect of reducing serum and/or increasing insulin on the stratification and secretion of aligned matrix by fourth- to fifth-passage bovine corneal stromal keratocytes. Medium concentrations of 0%, 1%, or 10% fetal bovine serum and 0% or 1% insulin-transferrin-selenium were investigated. High-resolution differential interference contrast microscopy, quick-freeze/deep-etch, and conventional transmission electron microscopy were used to monitor the evolution, morphology, and ultrastructure of the cell-matrix constructs. In a medium containing 1% each of serum and insulin-transferrin-selenium, stromal cells stratified and secreted abundant and locally aligned matrix, generating the thickest cell-matrix constructs (allowing handling with forceps). The results of this study have the potential to significantly advance the field of developmental functional engineering of load-bearing tissues by (i) elucidating cues that modulate in vitro cell secretion of organized matrix and (ii) establishing an optically accessible cell culture system for investigating the mechanism of cell secretion of aligned collagen fibrils.

Nanoscale topography-induced modulation of fundamental cell behaviors of rabbit corneal keratocytes, fibroblasts, and myofibroblasts

PURPOSE

Keratocyte-to-myofibroblast differentiation is a key factor in corneal wound healing. The purpose of this study was to determine the influence of environmental nanoscale topography on keratocyte, fibroblast, and myofibroblast cell behavior.

METHODS

Primary rabbit corneal keratocytes, fibroblasts, and myofibroblasts were seeded onto planar polyurethane surfaces with six patterned areas, composed of anisotropically ordered grooves and ridges with a 400-, 800-, 1200-, 1600-, 2000-, and 4000-nm pitch (pitch = groove + ridge width). After 24 hours cells were fixed, stained, imaged, and analyzed for cell shape and orientation. For migration studies, cells on each patterned surface were imaged every 10 minutes for 12 hours, and individual cell trajectories and migration rates were calculated.

RESULTS

Keratocytes, fibroblasts, and myofibroblasts aligned and elongated to pitch sizes larger than 1000 nm. A lower limit to the topographic feature sizes that the cells responded to was identified for all three phenotypes, with a transition zone around the 800- to 1200-nm pitch size. Fibroblasts and myofibroblasts migrated parallel to surface ridges larger than 1000 nm but lacked directional guidance on submicron and nanoscale topographic features and on planar surfaces. Keratocytes remained essentially immobile.

CONCLUSIONS

Corneal stromal cells elongated, aligned, and migrated, differentially guided by substratum topographic features. All cell types failed to respond to topographic features approximating the dimensions of individual stromal fibers. These findings contribute to our understanding of corneal stromal cell biology in health and disease and their interaction with biomaterials and their native extracellular matrix.

Growth factors and corneal epithelial wound healing

In this article, we briefly review recent findings in the effects of growth factors including the EGF family, KGF, HGF, IGF, insulin, and TGF-beta on corneal epithelial wound healing. We discuss the essential role of EGFR in inter-receptor cross-talk in response to wounding in corneal epithelium and bring forward a concept of "alarmins" to the field of wound healing research.

IGF-II and collagen expression by keratocytes during postnatal development

Keratocytes produce the extensive stromal matrix of the cornea during the late embryonic and neonatal time periods. We propose to test the hypothesis that their biosynthetic activity declines during this process. Keratocytes were isolated from corneas of 6-8-week-old rabbits and corneas of 1-2-year-old cows and their ability to proliferate and synthesize collagen in serum-free media was determined. Rabbit keratocyte cultures increased 38% in DNA content after one week and deposited collagen type I and IGF-II in the media. Bovine keratocyte cultures, in contrast, did not increase in DNA or produce detectable collagen and IGF-II. Bovine keratocytes cultured in media previously conditioned by rabbit keratocytes, however, increased 56% in DNA content, and deposited collagen type I into the media. Microarray analysis of mRNA from neonatal and adult mouse keratocytes was used to confirm these differences. Compared to adult mouse keratocytes, neonatal keratocytes showed high expression levels of IGF-I, IGF-II and collagen types III and V. Since previous studies showed that IGFs stimulate bovine keratocytes to proliferate and to synthesize procollagen type I, we therefore propose that the results of this study suggests that the IGFs may play an important role in regulating early corneal growth in vivo.

Increased stromal extracellular matrix synthesis and assembly by insulin activated bovine keratocytes cultured under agarose

Previously, pharmacological levels of insulin have been shown to stimulate the synthesis of normal corneal stromal collagen and proteoglycans by bovine keratocytes in culture. Here we compared insulin to physiological levels of IGF-I and found that IGF-I also stimulated the synthesis of these extracellular matrix components, but less than that of insulin. Keratocytes in monolayer culture secreted most of the collagen synthesized into the media in the form of procollagen, a precursor of collagen. We found that an overlay of 3% agarose on the keratocytes in culture enhanced the conversion of procollagen to collagen and increased the deposition of collagen and proteoglycans into the cell layer. The extracellular matrix associated with the keratocytes cultured under agarose exhibited a corneal stromal-like architecture. These results suggest that enhancing the conversion of procollagen to collagen is a key step in the formation of extracellular matrix by keratocytes in vitro. Agarose overlay of insulin activated keratocytes in culture is a useful model for studying corneal stromal extracellular matrix assembly in vitro.

Prelude to corneal tissue engineering - gaining control of collagen organization

By most standard engineering practice principles, it is premature to credibly discuss the "engineering" of a human cornea. A professional design engineer would assert that we still do not know what a cornea is (and correctly so), therefore we cannot possibly build one. The proof resides in the fact that there are no clinically viable corneas based on classical tissue engineering methods available. This is possibly because tissue engineering in the classical sense (seeding a degradable scaffolding with a population synthetically active cells) does not produce conditions which support the generation of organized tissue. Alternative approaches to the problem are in their infancy and include the methods which attempt to recapitulate development or to produce corneal stromal analogs de novo which require minimal remodeling. Nonetheless, tissue engineering efforts, which have been focused on producing the fundamental functional component of a cornea (organized alternating arrays of collagen or "lamellae"), may have already provided valuable new insights and tools relevant to development, growth, remodeling and pathologies associated with connective tissue in general. This is because engineers ask a fundamentally different question (How can that be done?) than do biological scientists (How is that done?). The difference in inquiry has prompted us to closely examine (and to mimic) development as well as investigate collagen physicochemical behavior so that we may exert control over organization both in cell culture (in vitro) and on the benchtop (de novo). Our initial results indicate that reproducing corneal stroma-like local and long-range organization of collagen may be simpler than we anticipated while controlling spacing and fibril morphology remains difficult, but perhaps not impossible in the (reasonably) near term.

Epidermal growth factor synergism with TGF-beta1 via PI-3 kinase activity in corneal keratocyte differentiation

PURPOSE

To investigate the action of epidermal growth factor (EGF) on corneal keratocyte differentiation and its effects in conjunction with transforming growth factor (TGF)-beta1.

METHODS

Rabbit corneal keratocytes (RCKs) were treated with EGF, TGF-beta1, or EGF plus TGF-beta1 in the presence or absence of inhibitors of EGF-receptor (EGF-R), neutralizing concentrations of EGF antibody and of signaling kinases for 2 days to 1 week. RCK differentiation to myofibroblasts was identified with anti-aldehyde dehydrogenase (ALDH)-1 and alpha-smooth muscle actin (alpha-SMA) antibodies. Cell proliferation was evaluated with anti-Ki-67 antibody. Extracellular matrix (ECM) components were assayed by immunochemistry and Western blot. Cell migration images were captured with a camera attached to the microscope, and the area of the wound was calculated using imaging software.

RESULTS

RCKs cultured in serum-free DMEM/F12 without frequent changes of medium maintained the phenotype for more than 1 month. EGF stimulated differentiation into a proto-myofibroblast phenotype with the loss of dendritic shape and the expression of alpha-SMA. Treatment with TGF-beta1 stimulated 12% of the cells to differentiate to defined myofibroblasts, but in the presence of EGF, TGF-beta1 induced 90% of RCKs to transform into myofibroblasts. Inhibition of EGF-R activation and of the phosphatidylinositol-3 kinase (PI-3K)/Akt-1 pathway prevented the action of EGF on TGF-beta1 cell differentiation. TGF-beta1 in the presence of EGF also increased cell migration, which is inhibited by blocking EGF-R activation.

CONCLUSIONS

These data show that EGF contributes to the differentiation and migration of myofibroblasts induced by TGF-beta1 through EGF-R activation and that it is an important modulator of wound healing and scar tissue formation.

IGF-II is present in bovine corneal stroma and activates keratocytes to proliferate in vitro

Extracts of bovine corneal stroma have been shown to activate keratocytes in culture to proliferate. We fractionated stromal extract on a column of Sephacryl S-300 and tested the fractions for mitogenic activity using cell culture and for the presence of IGF-II and its binding protein IGFBP-2 by Western blot. We found that the mitogenic activity in the extract separated into major and minor peaks and that immunologically detectable IGF-II and IGFBP-2 co-eluted with the minor peak. We also compared the effects of 10 ng IGF-II/ml on keratocytes in culture to that of 2 ng TGF-beta/ml over a 7-day culture period. We found that IGF-II and TGF-beta, alone or combined, increased both (3)H-thymidine incorporation and DNA content of the cultures. The phenotype of the cells was determined by using antibodies to alpha-SM (smooth muscle) actin, fibronectin, SPARC, lumican and keratocan in Western blots of cell layers of media. Keratocytes cultured in IGF-II expressed no alpha-SM actin or fibronectin, low levels of SPARC and high levels of lumican and keratocan, indicating a native phenotype. Keratocytes in TGF-beta expressed alpha-SM actin, fibronectin, SPARC and lumican, and expressed no or low levels of keratocan, indicating a myofibroblast phenotype. Keratocytes cultured in IGF-II plus TGF-beta, however, expressed alpha-SM actin, fibronectin, SPARC, lumican, and keratocan by day 7 of culture. The results of this study show that IGF-II to be present in the corneal stroma, to stimulate keratocyte proliferation while maintaining native phenotype and to override the TGF-beta mediated down regulation of keratocan production. The IGF-II in the stroma may serve as a mechanism to immediately activate keratocytes upon wounding and to ameliorate the scarring effects of TGF-beta.

Loss of alpha3(IV) collagen expression associated with corneal keratocyte activation

PURPOSE

To determine whether changes in the expression of type IV alpha1, alpha2, or alpha3 collagen isoforms are stringently associated with corneal stromal cell activation.

METHODS

Keratocytes isolated from rabbit corneal stroma by collagenase digestion were plated in serum-free or insulin-, bFGF/heparin sulfate (HS)-, TGF-beta1-, or fetal bovine serum (FBS)-supplemented DMEM/F12 medium. Expression of type IV collagen isoforms and keratan sulfate proteoglycans (KSPGs) was evaluated by immunocytochemical analysis, Western blot analysis, or both. Concentrations of mRNAs were estimated by quantitative RT-PCR using SYBR Green RT-PCR reagents.

RESULTS

Immunohistochemical analysis indicated that type IV alpha1, alpha2, and alpha3 collagens were expressed in normal rabbit corneal stroma and in keratocytes cultured in serum-free and insulin-supplemented media. However, alpha3(IV) collagen was not detectable in the regenerating stroma after photorefractive keratectomy (PRK) in rabbit or in corneal stromal cells cultured in media supplemented with FBS, bFGF/HS, or TGF-beta1. alpha3(IV) collagen mRNA levels were also diminished in the stromal cells cultured in these growth factor-supplemented media. KSPGs (lumican and keratocan) were expressed and secreted in serum-free medium. Although the expression of KSPGs was promoted by insulin, the expression and intracellular levels of lumican and keratocan mRNAs were downregulated by TGF-beta1 and FBS. bFGF/HS promoted the downregulation of intracellular keratocan but not lumican mRNA levels.

CONCLUSIONS

The loss in the expression of alpha3(IV) collagen is a stringent phenotypic change associated with activation of keratocytes in vivo and in vitro. This phenotypic change in activated corneal stromal cells is induced by bFGF/HS and by TGF-beta1, and it accompanies the downregulation of keratocan expression.