Optimisation of polymer scaffolds for retinal pigment epithelium (RPE) cell transplantation

Biomaterials used for tissue engineering of barrier-forming cell monolayers in the eye

Cell monolayers that form a barrier between two structures play an important role for the maintenance of tissue functionality. In the anterior portion of the eye, the corneal endothelium forms a barrier that controls fluid exchange between the aqueous humor of the anterior chamber and the corneal stroma. This monolayer is central in the pathogenesis of Fuchs endothelial corneal dystrophy (FECD). FECD is a common corneal disease, in which corneal endothelial cells deposit extracellular matrix that increases the thickness of its basal membrane (Descemet's membrane), and forms excrescences (guttae). With time, there is a decrease in endothelial cell density that generates vision loss. Transplantation of a monolayer of healthy corneal endothelial cells on a Descemet membrane substitute could become an interesting alternative for the treatment of this pathology. In the back of the eye, the retinal pigment epithelium (RPE) forms the blood-retinal barrier, controlling fluid exchange between the choriocapillaris and the photoreceptors of the outer retina. In the retinal disease dry age-related macular degeneration (dry AMD), deposits (drusen) form between the RPE and its basal membrane (Bruch's membrane). These deposits hinder fluid exchange, resulting in progressive RPE cell death, which in turn generates photoreceptor cell death, and vision loss. Transplantation of a RPE monolayer on a Bruch's membrane/choroidal stromal substitute to replace the RPE before photoreceptor cell death could become a treatment alternative for this eye disease. This review will present the different biomaterials that are proposed for the engineering of a monolayer of corneal endothelium for the treatment of FECD, and a RPE monolayer for the treatment of dry AMD.

Ocular application of electrospun materials for drug delivery and cellular therapies

The constraints of delivering conventional drugs, biologics and cell-based therapeutics to target ocular sites necessitate the fabrication of novel drug delivery systems to treat diverse ocular diseases. Conventional ocular drug delivery approaches are prone to low bioavailability, poor penetration and degradation of therapeutics, including cell-based therapies, leading to the need for frequent topical applications or intraocular injections. However, owing to their exceptional structural properties, nanofibrous and microfibrous electrospun materials have gained significant interest in ocular drug delivery and biomaterial applications. This review covers the recent developments of electrospun fibers for the delivery of drugs, biologics, cells, growth factors and tissue regeneration in treating ocular diseases. The insights from this review can provide a thorough understanding of the selection of materials for the fabrication of nano- and/or micro-fibrous systems for ocular applications, with a particular interest in achieving controlled drug release and cell therapy. A detailed modality for fabricating different types of nano- and micro-fibers produced from electrospinning and factors influencing generation are also discussed.

The Use of Polymer Blends in the Treatment of Ocular Diseases

The eye is an organ with limited drug access due to its anatomical and physiological barriers, and the usual forms of ocular administration are limited in terms of drug penetration, residence time, and bioavailability, as well as low patient compliance. Hence, therapeutic innovations in new drug delivery systems (DDS) have been widely explored since they show numerous advantages over conventional methods, besides delivering the content to the eye without interfering with its normal functioning. Polymers are usually used in DDS and many of them are applicable to ophthalmic use, especially biodegradable ones. Even so, it can be a hard task to find a singular polymer with all the desirable properties to deliver the best performance, and combining two or more polymers in a blend has proven to be more convenient, efficient, and cost-effective. This review was carried out to assess the use of polymer blends as DDS. The search conducted in the databases of Pubmed and Scopus for specific terms revealed that although the physical combination of polymers is largely applied, the term polymer blend still has low compliance.

Electrospun Nanofibers for Manipulating the Soft Tissue Regeneration

Soft tissue damage is a common clinical problem that affects the lives of a large number of patients all over the world. It is of great importance to develop functional...

Nano-Biomaterials for Retinal Regeneration

Nanoscience and nanotechnology have revolutionized key areas of environmental sciences, including biological and physical sciences. Nanoscience is useful in interconnecting these sciences to find new hybrid avenues targeted at improving daily life. Pharmaceuticals, regenerative medicine, and stem cell research are among the prominent segments of biological sciences that will be improved by nanostructure innovations. The present review was written to present a comprehensive insight into various emerging nanomaterials, such as nanoparticles, nanowires, hybrid nanostructures, and nanoscaffolds, that have been useful in mice for ocular tissue engineering and regeneration. Furthermore, the current status, future perspectives, and challenges of nanotechnology in tracking cells or nanostructures in the eye and their use in modified regenerative ophthalmology mechanisms have also been proposed and discussed in detail. In the present review, various research findings on the use of nano-biomaterials in retinal regeneration and retinal remediation are presented, and these findings might be useful for future clinical applications.

The Evolution of Fabrication Methods in Human Retina Regeneration

Optic nerve and retinal diseases such as age-related macular degeneration and inherited retinal dystrophies (IRDs) often cause permanent sight loss. Currently, a limited number of retinal diseases can be treated. Hence, new strategies are needed. Regenerative medicine and especially tissue engineering have recently emerged as promising alternatives to repair retinal degeneration and recover vision. Here, we provide an overview of retinal anatomy and diseases and a comprehensive review of retinal regeneration approaches. In the first part of the review, we present scaffold-free approaches such as gene therapy and cell sheet technology while in the second part, we focus on fabrication techniques to produce a retinal scaffold with a particular emphasis on recent trends and advances in fabrication techniques. To this end, the use of electrospinning, 3D bioprinting and lithography in retinal regeneration was explored.

Tissue Engineering Strategies for Retina Regeneration

The retina is a complex and fragile photosensitive part of the central nervous system which is prone to degenerative diseases leading to permanent vision loss. No proven treatment strategies exist to treat or reverse the degenerative conditions. Recent investigations demonstrate that cell transplantation therapies to replace the dysfunctional retinal pigment epithelial (RPE) cells and or the degenerating photoreceptors (PRs) are viable options to restore vision. Pluripotent stem cells, retinal progenitor cells, and somatic stem cells are the main cell sources used for cell transplantation therapies. The success of retinal transplantation based on cell suspension injection is hindered by limited cell survival and lack of cellular integration. Recent advances in material science helped to develop strategies to grow cells as intact monolayers or as sheets on biomaterial scaffolds for transplantation into the eyes. Such implants are found to be more promising than the bolus injection approach. Tissue engineering techniques are specifically designed to construct biodegradable or non-degradable polymer scaffolds to grow cells as a monolayer and construct implantable grafts. The engineered cell construct along with the extracellular matrix formed, can hold the cells in place to enable easy survival, better integration, and improved visual function. This article reviews the advances in the use of scaffolds for transplantation studies in animal models and their application in current clinical trials.

Optimisation of a Novel Bio-Substrate as a Treatment for Atrophic Age-Related Macular Degeneration

Atrophic age-related macular degeneration (AMD) is the most common form of AMD accounting for 90% of patients. During atrophic AMD the waste/exchange pathway between the blood supply (choroid) and the retinal pigment epithelium (RPE) is compromised. This results in atrophy and death of the RPE cells and subsequently the photoreceptors leading to central blindness. Although the mechanisms behind AMD are unknown, the growth of fatty deposits known as drusen, have been shown to play a role in the disease. There is currently no treatment or cure for atrophic AMD. Much research focuses on developing a synthetic substrate in order to transplant healthy cells to the native Bruch’s membrane (BM), however, the diseased native BM and related structures still leave potential for transplanted cells to succumb to disease. In this proof-of-concept work we electrospun poly(ethylene terephthalate) (PET) to fabricate a nanofibrous cytocompatible synthetic BM. The apical surface of the membrane was cultured with ARPE-19 cells and the underside was decorated with poly(lactic acid-co-glycolic acid) (PLGA) or poly(glycolic acid) (PGA) degradable nanoparticles by electrospraying. The membrane exhibited hydrophilicity, high tensile strength and structurally resembled the native BM. ARPE-19 cells were able to form a monolayer on the surface of the membrane and no cell invasion into the membrane was seen. The presence of both PLGA and PGA nanoparticles increased ARPE-19 cell metabolism but had no effect on cell viability. There was a decrease in pH of ARPE-19 cell culture media 7 days following culturing with the PLGA nanoparticles but this change was eliminated by 2 weeks; PGA nanoparticles had no effect on cell culture media pH. The fluorescent dye FITC was encapsulated into nanoparticles and showed sustained release from PLGA nanoparticles for 2 weeks and PGA nanoparticles for 1 day. Future work will focus on encapsulating biologically active moieties to target drusen. This could allow this novel bioactive substrate to be a potential treatment for atrophic AMD that would function two-fold: deliver the required monolayer of healthy RPE cells to the macula on a synthetic BM and remove diseased structures within the retina, restoring the waste/exchange pathway and preventing vision loss.

Bio-inspired human in vitro outer retinal models: Bruch's membrane and its cellular interactions

Retinal degenerative disorders, such as age-related macular degeneration (AMD), are one of the leading causes of blindness worldwide, however, treatments to completely stop the progression of these debilitating conditions are non-existent. Researchers require sophisticated models that can accurately represent the native structure of human retinal tissue to study these disorders. Current in vitro models used to study the retina are limited in their ability to fully recapitulate the structure and function of the retina, Bruch's membrane and the underlying choroid. Recent developments in the field of induced pluripotent stem cell technology has demonstrated the capability of retinal pigment epithelial cells to recapitulate AMD-like pathology. However, such studies utilise unsophisticated, bio-inert membranes to act as Bruch's membrane and support iPSC-derived retinal cells. This review presents a concise summary of the properties and function of the Bruch's membrane-retinal pigment epithelium complex, the initial pathogenic site of AMD as well as the current status for materials and fabrication approaches used to generate in vitro models of this complex tissue. Finally, this review explores required advances in the field of in vitro retinal modelling. STATEMENT OF SIGNIFICANCE: Retinal degenerative disorders such as age-related macular degeneration are worldwide leading causes of blindness. Previous attempts to model the Bruch's membrane-retinal pigment epithelial complex, the initial pathogenic site of age-related macular degeneration, have lacked the sophistication to elucidate valuable insights into disease mechanisms. Here we provide a detailed account of the morphological, physical and chemical properties of Bruch's membrane which may aid the fabrication of more sophisticated and physiologically accurate in vitro models of the retina, as well as various fabrication techniques to recreate this structure. This review also further highlights some recent advances in some additional challenging aspects of retinal tissue modelling including integrated fluid flow and photoreceptor alignment.

Development of an Anti-Inflammatory Drug-Incorporated Biomimetic Scaffold for Corneal Tissue Engineering

Purpose: The aim of this study was to design naproxen sodium (NS)-containing, biomimetic, porous poly(lactide-co-glycolide) (PLGA) scaffolds for regeneration of damaged corneal epithelium. Methods: NS-incorporated PLGA scaffolds were prepared using the emulsion freeze-drying method and then coated with collagen or poly-l-lysine. Porosity measurements of the scaffolds were performed by the gas adsorption/desorption method and the scaffolds demonstrated highly porous, open-cellular pore structures with pore sizes from 150 to 200 μm. Results: The drug loading efficiency of scaffolds was found to be higher than 84%, and about 90%-98% of NS was released at the end of 7 days with a fast drug release rate at the initial period of time and then in a slow and sustained manner. The corneal epithelial cells were isolated from New Zealand white rabbits. The obtained cells were seeded onto scaffolds and continued to increase during the time period of the study, indicating that the scaffolds might promote corneal epithelial cell proliferation without causing toxic effects for at least 10 days. Conclusions: The NS-loaded PLGA scaffolds exhibited a combination of controlled drug release and biomimetic properties that might be attractive for use in treatment of corneal damage both for controlled release and biomedical applications.

Regenerative medicine as a novel strategy for AMD treatment: a review

Age-related macular degeneration (AMD) is known as a major cause of irreversible blindness in elderly adults. The segment of the retina responsible for central vision damages in the disease process. Degeneration of retinal pigmented epithelium (RPE) cells, photoreceptors, and choriocapillaris associated with aging participate for visual loss. In 2010, AMD involved 6.6% of all blindness cases around the world. Some of the researches have evaluated the replacing of damaged RPE in AMD patients by using the cells from various sources. Today, the advancement of RPE differentiation or generation from stem cells has been gained, and currently, clinical trials are testing the efficiency and safety of replacing degenerated RPE with healthy RPE. However, the therapeutic success of RPE transplantation may be restricted unless the transplanted cells can be adhered, distributed and survive for long-term in the transplanted site without any infections. In recent years a variety of scaffold types were used as a carrier for RPE transplantation and AMD treatment. In this review, we have discussed types of scaffolds; natural or synthetic, solid or hydrogel and their results in RPE replacement. Eventually, our aim is highlighting the novel and best scaffold carriers that may have potentially promoting the efficacy of RPE transplantation.

Utilizing Recombinant Spider Silk Proteins To Develop a Synthetic Bruch's Membrane for Modeling the Retinal Pigment Epithelium

Spider silks are intriguing biomaterials that have a high potential as innovative biomedical processes and devices. The intent of this study was to evaluate the capacity of recombinant spider silk proteins (rSSps) as a synthetic Bruch's membrane. Nonporous silk membranes were prepared with comparable thicknesses (<10 μm) to that of native Bruch's membrane. Biomechanical characterization was performed prior to seeding cells. The ability of RPE cells (ARPE-19) to attach and grow on the membranes was then evaluated with bright-field and electron microscopy, intracellular DNA quantification, and immunocytochemical staining (ZO-1 and F-actin). Controls were cultured on permeable Transwell support membranes and characterized with the same methods. A size-dependent permeability assay, using FITC-dextran, was used to determine cell-membrane barrier function. Compared to Transwell controls, RPE cells cultured on rSSps membranes developed more native-like "cobblestone" morphologies, exhibited higher intracellular DNA content, and expressed key organizational proteins more consistently. Comparisons of the membranes to native structures revealed that the silk membranes exhibited equivalent thicknesses, biomechanical properties, and barrier functions. These findings support the use of recombinant spider silk proteins to model Bruch's membrane and develop more biomimetic retinal models.

Pluripotent Stem Cells for Retinal Tissue Engineering: Current Status and Future Prospects

The retina is a very fine and layered neural tissue, which vitally depends on the preservation of cells, structure, connectivity and vasculature to maintain vision. There is an urgent need to find technical and biological solutions to major challenges associated with functional replacement of retinal cells. The major unmet challenges include generating sufficient numbers of specific cell types, achieving functional integration of transplanted cells, especially photoreceptors, and surgical delivery of retinal cells or tissue without triggering immune responses, inflammation and/or remodeling. The advances of regenerative medicine enabled generation of three-dimensional tissues (organoids), partially recreating the anatomical structure, biological complexity and physiology of several tissues, which are important targets for stem cell replacement therapies. Derivation of retinal tissue in a dish creates new opportunities for cell replacement therapies of blindness and addresses the need to preserve retinal architecture to restore vision. Retinal cell therapies aimed at preserving and improving vision have achieved many improvements in the past ten years. Retinal organoid technologies provide a number of solutions to technical and biological challenges associated with functional replacement of retinal cells to achieve long-term vision restoration. Our review summarizes the progress in cell therapies of retina, with focus on human pluripotent stem cell-derived retinal tissue, and critically evaluates the potential of retinal organoid approaches to solve a major unmet clinical need—retinal repair and vision restoration in conditions caused by retinal degeneration and traumatic ocular injuries. We also analyze obstacles in commercialization of retinal organoid technology for clinical application.

Development of a Graphene Oxide-Incorporated Polydimethylsiloxane Membrane with Hexagonal Micropillars

Several efforts have been made on the development of bioscaffolds including the polydimethylsiloxane (PDMS) elastomer for supporting cell growth into stable sheets. However, PDMS has several disadvantages, such as intrinsic surface hydrophobicity and mechanical strength. Herein, we generated a novel PDMS-based biomimetic membrane by sequential modifications of the PMDS elastomer with graphene oxide (GO) and addition of a hexagonal micropillar structure at the bottom of the biomembrane. GO was initially homogenously mixed with pure PDMS and then was further coated onto the upper surface of the resultant PDMS. The elastic modulus and hydrophilicity were significantly improved by such modifications. In addition, the development of hexagonal micropillars with smaller diameters largely improved the ion permeability and increased the motion resistance. We further cultured retinal pigment epithelial (RPE) cells on the surface of this modified PDMS biomembrane and assayed its biocompatibility. Remarkably, the GO incorporation and coating exhibited beneficial effect on the cell growth and the new formation of tight junctions in RPE cells. Taken together, this GO-modified PDMS scaffold with polyhexagonal micropillars may be utilized as an ideal cell sheet and adaptor for cell cultivation and can be used in vivo for the transplantation of cells such as RPE cells.

A convenient protocol for establishing a human cell culture model of the outer retina

The retinal pigment epithelium (RPE) plays a key role in the pathogenesis of several blinding retinopathies. Alterations to RPE structure and function are reported in Age-related Macular Degeneration, Stargardt and Best disease as well as pattern dystrophies. However, the precise role of RPE cells in disease aetiology remains incompletely understood. Many studies into RPE pathobiology have utilised animal models, which only recapitulate limited disease features. Some studies are also difficult to carry out in animals as the ocular space remains largely inaccessible to powerful microscopes. In contrast, in-vitro models provide an attractive alternative to investigating pathogenic RPE changes associated with age and disease. In this article we describe the step-by-step approach required to establish an experimentally versatile in-vitro culture model of the outer retina incorporating the RPE monolayer and supportive Bruch's membrane (BrM). We show that confluent monolayers of the spontaneously arisen human ARPE-19 cell-line cultured under optimal conditions reproduce key features of native RPE. These models can be used to study dynamic, intracellular and extracellular pathogenic changes using the latest developments in microscopy and imaging technology. We also discuss how RPE cells from human foetal and stem-cell derived sources can be incorporated alongside sophisticated BrM substitutes to replicate the aged/diseased outer retina in a dish. The work presented here will enable users to rapidly establish a realistic in-vitro model of the outer retina that is amenable to a high degree of experimental manipulation which will also serve as an attractive alternative to using animals. This in-vitro model therefore has the benefit of achieving the 3Rs objective of reducing and replacing the use of animals in research. As well as recapitulating salient structural and physiological features of native RPE, other advantages of this model include its simplicity, rapid set-up time and unlimited scope for detailed single-cell resolution and matrix studies.

Decellularized matrices in regenerative medicine

Of all biologic matrices, decellularized extracellular matrix (dECM) has emerged as a promising tool used either alone or when combined with other biologics in the fields of tissue engineering or regenerative medicine - both preclinically and clinically. dECM provides a native cellular environment that combines its unique composition and architecture. It can be widely obtained from native organs of different species after being decellularized and is entitled to provide necessary cues to cells homing. In this review, the superiority of the macro- and micro-architecture of dECM is described as are methods by which these unique characteristics are being harnessed to aid in the repair and regeneration of organs and tissues. Finally, an overview of the state of research regarding the clinical use of different matrices and the common challenges faced in using dECM are provided, with possible solutions to help translate naturally derived dECM matrices into more robust clinical use.

STATEMENT OF SIGNIFICANCE

Ideal scaffolds mimic nature and provide an environment recognized by cells as proper. Biologically derived matrices can provide biological cues, such as sites for cell adhesion, in addition to the mechanical support provided by synthetic matrices. Decellularized extracellular matrix is the closest scaffold to nature, combining unique micro- and macro-architectural characteristics with an equally unique complex composition. The decellularization process preserves structural integrity, ensuring an intact vasculature. As this multifunctional structure can also induce cell differentiation and maturation, it could become the gold standard for scaffolds.

Collective adhesion and displacement of retinal progenitor cells upon extracellular matrix substrates of transplantable biomaterials

Strategies to replace retinal photoreceptors lost to damage or disease rely upon the migration of replacement cells transplanted into sub-retinal spaces. A significant obstacle to the advancement of cell transplantation for retinal repair is the limited migration of transplanted cells into host retina. In this work, we examine the adhesion and displacement responses of retinal progenitor cells on extracellular matrix substrates found in retina as well as widely used in the design and preparation of transplantable scaffolds. The data illustrate that retinal progenitor cells exhibit unique adhesive and displacement dynamics in response to poly-l-lysine, fibronectin, laminin, hyaluronic acid, and Matrigel. These findings suggest that transplantable biomaterials can be designed to improve cell integration by incorporating extracellular matrix substrates that affect the migratory behaviors of replacement cells.

Ex-vivo models of the Retinal Pigment Epithelium (RPE) in long-term culture faithfully recapitulate key structural and physiological features of native RPE

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Damage to the Retinal Pigment Epithelium (RPE) is a key feature of retinopathy.

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We describe 2 substrates which support RPE cultures for long-term studies.

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Substrates were; a polyester transwell membrane and a novel electrospun scaffold.

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Both support RPE cultures with structural and functional features of native RPE.

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Electrospun scaffolds may be better for studying some disease-linked RPE changes.

The Retinal Pigment Epithelium (RPE) forms the primary site of pathology in several blinding retinopathies. RPE cultures are being continuously refined so that dynamic disease processes in this important monolayer can be faithfully studied outside the eye over longer periods. The RPE substrate, which mimics the supportive Bruch’s membrane (BrM), plays a key role in determining how well in-vitro cultures recapitulate native RPE cells. Here, we evaluate how two different types of BrM substrates; (1) a commercially-available polyester transwell membrane, and (2) a novel electrospun scaffold developed in our laboratory, could support the generation of realistic RPE tissues in culture. Our findings reveal that both substrates were capable of supporting long-lasting RPE monolayers with structural and functional specialisations of in-situ RPE cells. These cultures were used to study autofluorescence and barrier formation, as well as activities such as outer-segment internalisation/trafficking and directional secretion of key proteins; the impairment of which underlies retinal disease. Hence, both substrates fulfilled important criteria for generating authentic in-vitro cultures and act as powerful tools to study RPE pathophysiology. However, RPE grown on electrospun scaffolds may be better suited to studying complex RPE-BrM interactions such as the formation of drusen-like deposits associated with early retinal disease.

Gelatin-based 3D conduits for transdifferentiation of mesenchymal stem cells into Schwann cell-like phenotypes

In this study, gelatin-based 3D conduits with three different microstructures (nanofibrous, macroporous and ladder-like) were fabricated for the first time via combined molding and thermally induced phase separation (TIPS) technique for peripheral nerve regeneration. The effects of conduit microstructure and mechanical properties on the transdifferentiation of bone marrow-derived mesenchymal stem cells (MSCs) into Schwann cell (SC) like phenotypes were examined to help facilitate neuroregeneration and understand material-cell interfaces. Results indicated that 3D macroporous and ladder-like structures enhanced MSC attachment, proliferation and spreading, creating interconnected cellular networks with large numbers of viable cells compared to nanofibrous and 2D-tissue culture plate counterparts. 3D-ladder-like conduit structure with complex modulus of ∼0.4×10⁶Pa and pore size of ∼150μm provided the most favorable microenvironment for MSC transdifferentiation leading to ∼85% immunolabeling of all SC markers. On the other hand, the macroporous conduits with complex modulus of ∼4×10⁶Pa and pore size of ∼100μm showed slightly lower (∼65% for p75, ∼75% for S100 and ∼85% for S100β markers) immunolabeling. Transdifferentiated MSCs within 3D-ladder-like conduits secreted significant amounts (∼2.5pg/mL NGF and ∼0.7pg/mL GDNF per cell) of neurotrophic factors, while MSCs in macroporous conduits released slightly lower (∼1.5pg/mL NGF and 0.7pg/mL GDNF per cell) levels. PC12 cells displayed enhanced neurite outgrowth in media conditioned by conduits with transdifferentiated MSCs. Overall, conduits with macroporous and ladder-like 3D structures are promising platforms in transdifferentiation of MSCs for neuroregeneration and should be further tested in vivo.

STATEMENT OF SIGNIFICANCE

This manuscript focuses on the effect of microstructure and mechanical properties of gelatin-based 3D conduits on the transdifferentiation of mesenchymal stem cells to Schwann cell-like phenotypes. This work builds on our recently accepted manuscript in Acta Biomaterialia focused on multifunctional 2D films, and focuses on 3D microstructured conduits designed to overcome limitations of current strategies to facilitate peripheral nerve regeneration. The comparison between conduits fabricated with nanofibrous, macroporous and ladder-like microstructures showed that the ladder-like conduits showed the most favorable environment for MSC transdifferentiation to Schwann-cell like phenotypes, as seen by both immunolabeling as well as secretion of neurotrophic factors. This work demonstrates the importance of controlling the 3D microstructure to facilitate tissue engineering strategies involving stem cells that can serve as promising approaches for peripheral nerve regeneration.

Polarized, Cobblestone, Human Retinal Pigment Epithelial Cell Maturation on a Synthetic PEG Matrix

Cell attachment is essential for the growth and polarization of retinal pigment epithelial (RPE) cells. Currently, surface coatings derived from biological proteins are used as the gold standard for cell culture. However, downstream processing and purification of these biological products can be cumbersome and expensive. In this study, we constructed a library of chemically modified nanofibers to mimic the Bruch's membrane of the retinal pigment epithelium. Using atmospheric-pressure plasma-induced graft polymerization with a high-throughput screening platform to modify the nanofibers, we identified three polyethylene glycol (PEG)-grafted nanofiber surfaces (PEG methyl ether methacrylate, n = 4, 8, and 45) from a library of 62 different surfaces as favorable for RPE cell attachment, proliferation, and maturation in vitro with cobblestone morphology. Compared with the biologically derived culture matrices such as vitronectin-based peptide Synthemax, our newly discovered synthetic PEG surfaces exhibit similar growth and polarization of retinal pigment epithelial (RPE) cells. However, they are chemically defined, are easy to synthesize on a large scale, are cost-effective, are stable with long-term storage capability, and provide a more physiologically accurate environment for RPE cell culture. To our knowledge, no one has reported that PEG derivatives directly support attachment and growth of RPE cells with cobblestone morphology. This study offers a unique PEG-modified 3D cell culture system that supports RPE proliferation, differentiation, and maturation with cobblestone morphology, providing a new avenue for RPE cell culture, disease modeling, and cell replacement therapy.

Nanofiber Scaffold-Based Tissue-Engineered Retinal Pigment Epithelium to Treat Degenerative Eye Diseases

Clinical-grade manufacturing of a functional retinal pigment epithelium (RPE) monolayer requires reproducing, as closely as possible, the natural environment in which RPE grows. In vitro, this can be achieved by a tissue engineering approach, in which the RPE is grown on a nanofibrous biological or synthetic scaffold. Recent research has shown that nanofiber scaffolds perform better for cell growth and transplantability compared with their membrane counterparts and that the success of the scaffold in promoting cell growth/function is not heavily material dependent. With these strides, the field has advanced enough to begin to consider implementation of one, or a combination, of the tissue engineering strategies discussed herein. In this study, we review the current state of tissue engineering research for in vitro culture of RPE/scaffolds and the parameters for optimal scaffold design that have been uncovered during this research. Next, we discuss production methods and manufacturers that are capable of producing the nanofiber scaffolds in such a way that would be biologically, regulatory, clinically, and commercially viable. Then, a discussion of how the scaffolds could be characterized, both morphologically and mechanically, to develop a testing process that is viable for regulatory screening is performed. Finally, an example of a tissue-engineered RPE/scaffold construct is given to provide the reader a framework for understanding how these pieces could fit together to develop a tissue-engineered RPE/scaffold construct that could pass regulatory scrutiny and can be commercially successful.

Honeycomb porous films as permeable scaffold materials for human embryonic stem cell-derived retinal pigment epithelium

Age-related macular degeneration (AMD) is a leading cause of blindness in developed countries, characterised by the degeneration of the retinal pigment epithelium (RPE), a pigmented cell monolayer that closely interacts with the photoreceptors. RPE transplantation is thus considered a very promising therapeutic option to treat this disease. In this work, porous honeycomb-like films are for the first time investigated as scaffold materials for human embryonic stem cell-derived retinal pigment epithelium (hESC-RPE). By changing the conditions during film preparation, it was possible to produce films with homogeneous pore distribution and adequate pore size (∼3-5 µm), that is large enough to ensure high permeability but small enough to enable cell adherence and spreading. A brief dip-coating procedure with collagen type IV enabled the homogeneous adsorption of the protein to the walls and bottom of pores, increasing the hydrophilicity of the surface. hESC-RPE adhered and proliferated on all the collagen-coated materials, regardless of small differences in pore size. The differentiation of hESC-RPE was confirmed by the detection of specific RPE protein markers. These results suggest that the porous honeycomb films can be promising candidates for hESC-RPE tissue engineering, importantly enabling the free flow of ions and molecules across the material. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 104A: 1646-1656, 2016.

Behavior of a Spontaneously Arising Human Retinal Pigment Epithelial Cell Line Cultivated on Thin Alginate Film

Purpose:

A cell line spontaneously derived from human retinal pigment epithelium (hRPE) was cultured on alginate film gelatinized with different concentrations of neurobasal cell culture medium (NCCM) to assess its growth and morphological behavior on this naturally occurring polysaccharide.

Methods:

Neonatal human globes were used to isolate hRPE cells. They were cultured in Dulbecco's modified Eagle’s-medium-and-Ham’s-F12-medium-(DMEM/F12) supplemented with 10% fetal bovine serum (FBS). Cultures were continuously studied using phase contrast microscopy. After the nineth passage, cells were characterized through immunocytochemical analysis for Oct4, Chx10, and Pax6 and Ki67 markers. In each well of a 6-well microplate, 1 and 2% weight/volume (w/v) alginate in deionized water was added and gelatinized using 1× and 10× NCCM. hRPE cells were cultured at a density of 2 × 105 cells/well in alginate-coated microplates. After 5 days, hRPE colonies were harvested and re-plated on polystyrene substrates. Morphology and growth of hRPE cultures were determined during the next 2 weeks.

Results:

The first few passages of the cultures were purely hRPE cells that revealed typical morphological features of the pigmented epithelium. They made spaces, devoid of cells, between hRPE cell monolayer and fill in the unoccupied spaces. They grew faster than native RPE cells and rapidly overgrew. Immunocytochemical test revealed that the founded cells expressed Chx10, Pax6, Ki67 and Oct4. The hRPE cells survived unlimitedly on alginate film and formed giant adjoining colonies. After re-plating, hRPE colonies adhered quickly on polystyrene and displayed native hRPE morphological features.

Conclusion:

Alginate film can support the survival and growth of hRPE cells and induce the cells to re-organize in tissue-like structures.

Surface Modified Biodegradable Electrospun Membranes as a Carrier for Human Embryonic Stem Cell-Derived Retinal Pigment Epithelial Cells

Human embryonic stem cell-derived retinal pigment epithelial (hESC-RPE) cells are currently undergoing clinical trials to treat retinal degenerative diseases. Transplantation of hESC-RPE cells in conjuction with a supportive biomaterial carrier holds great potential as a future treatment for retinal degeneration. However, there has been no such biodegradable material that could support the growth and maturation of hESC-RPE cells so far. The primary aim of this work was to create a thin porous poly (L-lactide-co-caprolactone) (PLCL) membrane that could promote attachment, proliferation, and maturation of the hESC-RPE cells in serum-free culture conditions. The PLCL membranes were modified by atmospheric pressure plasma processing and coated with collagen IV to enhance cell growth and maturation. Permeability of the membranes was analyzed with an Ussing chamber system. Analysis with scanning electron microscopy, contact angle measurement, atomic force microscopy, and X-ray photoelectron spectroscopy demonstrated that plasma surface treatment augments the surface properties of the membrane, which enhances the binding and conformation of the protein. Cell proliferation assays, reverse transcription-polymerase chain reaction, indirect immunofluoresence staining, trans-epithelial electrical resistance measurements, and in vitro phagocytosis assay clearly demonstrated that the plasma treated PLCL membranes supported the adherence, proliferation, maturation and functionality of hESC-RPE cells in serum-free culture conditions. Here, we report for the first time, how PLCL membranes can be modified with atmospheric pressure plasma processing to enable the formation of a functional hESC-RPE monolayer on a porous biodegradable substrate, which have a potential as a tissue-engineered construct for regenerative retinal repair applications.

Porous poly(ε-caprolactone) scaffolds for retinal pigment epithelium transplantation

PURPOSE

Retinal pigment epithelium (RPE) transplantation is a promising strategy for the treatment of dry age-related macular degeneration (AMD). However, previous attempts at subretinal RPE cell transplantation have experienced limited success due to poor adhesion, organization, and function on aged or diseased Bruch's membrane. Instead, cell-based strategies may benefit from a synthetic scaffold that mimics the functions of healthy Bruch's membrane to promote the formation of a functional RPE monolayer while maintaining metabolite exchange between the vasculature and outer retina.

METHODS

This study evaluated the behavior of human RPE on nanopatterned porous poly(ε-caprolactone) (PCL) film as a potential scaffold for therapeutic transplantation. Fetal human RPE (fhRPE) was cultured on porous PCL, nonporous PCL, or Costar porous polyester transwells for up to 8 weeks and assessed using light microscopy, fluorescent microscopy, transepithelial resistance, quantitative PCR, ELISAs, and phagocytosis assays.

RESULTS

fhRPE on porous PCL displayed improved markers of maturity and function compared with both porous polyester transwells and nonporous PCL, including pigmentation, increased cell density, superior barrier function, up-regulation of RPE-specific genes, and polarized growth factor secretion.

CONCLUSIONS

This study indicates that porous PCL is an attractive scaffold for RPE transplantation. In addition to being biocompatible with the subretinal space, porous PCL also allows for trans-scaffold metabolite transport and significantly improves RPE cell behavior compared to nonporous PCL or porous polyester transwells.

Current treatment limitations in age-related macular degeneration and future approaches based on cell therapy and tissue engineering

Age-related macular degeneration (AMD) is the leading cause of blindness in the Western world. With an ageing population, it is anticipated that the number of AMD cases will increase dramatically, making a solution to this debilitating disease an urgent requirement for the socioeconomic future of the European Union and worldwide. The present paper reviews the limitations of the current therapies as well as the socioeconomic impact of the AMD. There is currently no cure available for AMD, and even palliative treatments are rare. Treatment options show several side effects, are of high cost, and only treat the consequence, not the cause of the pathology. For that reason, many options involving cell therapy mainly based on retinal and iris pigment epithelium cells as well as stem cells are being tested. Moreover, tissue engineering strategies to design and manufacture scaffolds to mimic Bruch's membrane are very diverse and under investigation. Both alternative therapies are aimed to prevent and/or cure AMD and are reviewed herein.

Montmorillonite clay based polyurethane nanocomposite as substrate for retinal pigment epithelial cell growth

The subretinal transplantation of retinal pigment epithelial cells (RPE cells) grown on polymeric supports may have interest in retinal diseases affecting RPE cells. In this study, montmorillonite based polyurethane nanocomposite (PU-NC) was investigated as substrate for human RPE cell growth (ARPE-19 cells). The ARPE-19 cells were seeded on the PU-NC, and cell viability, proliferation and differentiation were investigated. The results indicated that ARPE-19 cells attached, proliferated onto the PU-NC, and expressed occludin. The in vivo ocular biocompatibility of the PU-NC was assessed by using the HET-CAM; and through its implantation under the retina. The direct application of the nanocomposite onto the CAM did not compromise the vascular tissue in the CAM surface, suggesting no ocular irritancy of the PU-NC film. The nanocomposite did not elicit any inflammatory response when implanted into the eye of rats. The PU-NC may have potential application as a substrate for RPE cell transplantation.

Prospects of Stem Cells for Retinal Diseases

Retinal diseases, including glaucoma, retinitis pigmentosa, diabetic retinopathy, and age-related macular degeneration, are the leading causes of irreversible visual impairment and blindness in developed countries. Traditional and current treatment regimens are based on surgical or medical interventions to slow down the disease progression. However, the number of retinal cells would continue to diminish, and the diseases could not be completely cured. There is an emerging role of stem cells in retinal research. The stem cell therapy on retinal diseases is based on 2 theories: cell replacement therapy and neuroprotective effect. The former hypothesizes that new retinal cells could be regenerated from stem cells to substitute the damaged cells in the diseased retina, whereas the latter believes that the paracrine effects of stem cells modulate the microenvironments of the diseased retina so as to protect the retinal neurons. This article summarizes the choice of stem cells in retinal research. Moreover, the current progress of retinal research on stem cells and the clinical applications of stem cells on retinal diseases are reviewed. In addition, potential challenges and future prospects of retinal stem cell research are discussed.

Novel gradient casting method provides high-throughput assessment of blended polyester poly(lactic-co-glycolic acid) thin films for parameter optimization

Pure polymer films cannot meet the diverse range of controlled release and material properties demanded for the fabrication of medical implants or other devices. Additives are added to modulate and optimize thin films for the desired qualities. To characterize the property trends that depend on additive concentration, an assay was designed which involved casting a single polyester poly(lactic-co-glycolic acid) (PLGA) film that blends a linear gradient of any PLGA-soluble additive desired. Four gradient PLGA films were produced by blending polyethylene glycol or the more hydrophobic polypropylene glycol. The films were made using a custom glass gradient maker in conjunction with a 180 cm film applicator. These films were characterized in terms of thickness, percent additive, total polymer (PLGA+additive), and controlled drug release using drug-like fluorescent molecules such as coumarin 6 (COU) or fluorescein diacetate (FDAc). Material properties of elongation and modulus were also accessed. Linear gradients of additives were readily generated, with phase separation being the limiting factor. Additive concentration had a Pearson's correlation factor (R) of >0.93 with respect to the per cent total release after 30 days for all gradients characterized. Release of COU had a near zero-order release over the same time period, suggesting that coumarin analogs may be suitable for use in PLGA/polyethylene glycol or PLGA/polypropylene glycol matrices, with each having unique material properties while allowing tuneable drug release. The gradient casting method described has considerable potential in offering higher throughput for optimizing film or coating material properties for medical implants or other devices.

Ophthalmic transplantology: posterior segment of the eye--part II

Background

Transplants of the retina are among the new strategies being used in the treatment of genetic and degenerative macular diseases. Moreover, various cell cultures are being tested to treat retinal disorders.

Material/Methods

Literature dated from 2004 to 2011 was comprehensively examined via Medline and PubMed searches for the following terms: auto-, homo-, heterologous transplantation, retina, stem cells, cultivated cells.

Results

Tissue and cell therapy of retinal diseases are reviewed, including full-thickness retina/retinal pigment epithelium (RPE)/choroid graft; full and partial thickness RPE/choroid complex grafts; RPE/Bruch membrane complex graft; and RPE, iris pigment epithelium and stem cell grafts. Recommendations for transplants, as well as the benefits and weaknesses of specific techniques in retina transplants, are discussed.

Conclusions

Auto- and allogenic transplants of a full or partial thickness retina/RPE/Bruch membrane/choroid complex represent an alternative treatment offered to patients with some macular diseases. Stem cell transplantation to reconstruct and regenerate the macula requires further biomolecular and animal research studies.

Advances in Retinal Tissue Engineering

Retinal degenerations cause permanent visual loss and affect millions world-wide. Current treatment strategies, such as gene therapy and anti-angiogenic drugs, merely delay disease progression. Research is underway which aims to regenerate the diseased retina by transplanting a variety of cell types, including embryonic stem cells, fetal cells, progenitor cells and induced pluripotent stem cells. Initial retinal transplantation studies injected stem and progenitor cells into the vitreous or subretinal space with the hope that these donor cells would migrate to the site of retinal degeneration, integrate within the host retina and restore functional vision. Despite promising outcomes, these studies showed that the bolus injection technique gave rise to poorly localized tissue grafts. Subsequently, retinal tissue engineers have drawn upon the success of bone, cartilage and vasculature tissue engineering by employing a polymeric tissue engineering approach. This review will describe the evolution of retinal tissue engineering to date, with particular emphasis on the types of polymers that have routinely been used in recent investigations. Further, this review will show that the field of retinal tissue engineering will require new types of materials and fabrication techniques that optimize the survival, differentiation and delivery of retinal transplant cells.