Models for eye disorders
Pluripotent stem cell-based models to investigate retinal pigmented epithelium function and disease

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Animal models do not always recapitulate or translate to human diseases. Pluripotent stem cells provide a self-renewing resource to generate mature cell types, including those of the retina. Furthermore, induced pluripotent stem cells derived from patients with genetic diseases can be differentiated into specialized cells to create in vitro disease models with precise pathogenic genotypes. Recent progress in deriving functional RPE cells from these disease-carrying stem cells in vitro can provide insights into disease pathobiology, thus providing a platform for discovery of new pharmacological interventions.

Introduction

The retinal pigmented epithelium (RPE) forms a monolayer of highly specialized cells located between the neural retina and the choroid. The RPE is involved in a large number of processes critical to visual function and the health of the photoreceptors. Amongst its crucial functions is the constant engulfment and degradation of photoreceptor outer segments and the recycling of visual pigment products required for the proper functioning of the photoreceptors. The outer blood-retinal barrier is formed by the RPE and controls the exchange of fluid and molecules between the fenestrated capillaries of the choriocapillaris and the neural retina. Melanin granules, which give the RPE its characteristic pigmentation, absorb radiant energy and reduce the light scatter resulting in a better image received by the retina. With age, there is a gradual loss of RPE and changes in surviving RPE cell appearance and metabolism. Impairment in critical functions has severe consequences for the photoreceptors whose survival is dependent on a healthy RPE. Dysfunction in the RPE is thought to be integral in the development of age-related macular degeneration (AMD), the common cause of severe vision loss in Western communities [1].

Whilst the exact pathogenic mechanisms involved in AMD are not fully understood, the RPE is considered central. Understanding the pathogenic mechanisms is clearly crucial if we are to develop treatments to reduce the burden of this disease as our population ages. In addition, less frequent but often more devastating, inherited retinal dystrophies can also result from mutations within genes involved in pathways involving the RPE. One such dystrophy involves RPE65, a RPE specific protein involved in the visual cycle. Mutations in this gene result in severe, childhood onset blindness [2]. These disorders provide an impetus for research into effective modelling of these diseases to allow for greater understanding of disease pathogenesis and development of therapeutics. In this review we will discuss how stem cells can be used to model diseases involving the RPE.

Section snippets

Animal models

With regard to AMD, there is no good animal model to use to investigate the entire disease, although there are some rodent models that address specific pathology such as the thickened Bruch's membrane in the Apolipoprotein E mutant mouse model [3]. The lack of a macula in rodent models makes studying retinal diseases that particularly target the macular region, such as AMD, not ideal in these animals. Animal models are useful when studying some basic mutations of RPE genes, such as RPE65, and

Introduction to pluripotent stem cells

Human embryonic stem cells (hESCs) isolated from pre-implantation blastocysts are capable of forming any cell type found in the adult body, a characteristic known as pluripotency [13], [14]. In addition, hESCs can be cultured indefinitely in vitro without undergoing a restriction in pluripotency [13], [14], [15]. Initial derivation of hESCs and their subsequent differentiation provided an opportunity for the study of human development and as a potential source of cells for therapeutic

Characteristics of stem cell-derived RPE cells

In order to confidently utilise RPE cells differentiated from pluripotent hESCs and iPSCs (hESC-RPE and iPSC-RPE respectively) for experimentation, detailed analysis is required to determine their functional characteristics in comparison with primary adult or foetal RPE (fRPE) cells as well as with established retinal cell lines such as ARPE-19. Early transcriptome analyses demonstrate that hESC-RPE cells, derived from spontaneous differentiation, share greater similarity to fRPE cells than to

How stem cells can be used to model dysfunction of the RPE – disease in a dish

To successfully model any disease utilising iPSCs, three criteria must be met. Firstly, appropriate controls need to be utilised. Secondly, an efficient and reproducible differentiation method is required to generate cells of interest from iPSCs. Lastly, phenotypic differences need to be identified between diseased and control cells. Early disease modelling attempts used existing hESCs and iPSCs as unaffected controls for diseased iPSCs; however, it is unclear whether many of these existing

Transplantation and clinical trials

Given the potential of hESC- and iPSC-derived cells for therapeutic applications, the relative ease of differentiating these to RPE cells and the accessibility of the eye, there has been some progress towards using hESC- and iPSC-RPE cells in a clinical setting. Preclinical studies of both cell sources indicate that RPE cells could be immunologically tolerated (with immunosuppression) when transplanted in either rat models of retinal degeneration or mouse models of retinitis pigmentosa [17],

Conclusions

Pluripotent stem cells can readily differentiate into RPE cells that resemble primary RPE in both gene expression and function. As iPSCs can be derived from patients with genetic diseases affecting RPE function, they provide a potentially inexhaustible supply of cells with disease-specific genotypes and phenotypes that, with careful experimentation using known functions of native RPE cells, can provide a basis for both understanding disease biology and drug discovery. As such, these iPSC- and

Acknowledgements

This work was supported by a National Health and Medical Research Council (NHMRC) Practitioner Fellowship (RG, #529905), a NHMRC Career Development Award Fellowship (AP, #1004164), a NHMRC-CSL Gustav Nossal postgraduate research scholarship (DC, #1055499), a National Stem Cell Foundation of Australia Research Support grant (KD, AP), a NHMRC Project Grant (#1059369), and an Ophthalmic Research Institute of Australia/RANZCO Eye Foundation Grant. CERA receives Operational Infrastructure Support

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