RPE-derived factors modulate photoreceptor differentiation: a possible role in the retinal stem cell niche

ARPE-19 conditioned medium promotes neural differentiation of adipose-derived mesenchymal stem cells

BACKGROUND

Adipose-derived stem cells (ASCs) have been increasingly explored for cell-based medicine because of their numerous advantages in terms of easy availability, high proliferation rate, multipotent differentiation ability and low immunogenicity. In this respect, they have been widely investigated in the last two decades to develop therapeutic strategies for a variety of human pathologies including eye disease. In ocular diseases involving the retina, various cell types may be affected, such as Müller cells, astrocytes, photoreceptors and retinal pigment epithelium (RPE), which plays a fundamental role in the homeostasis of retinal tissue, by secreting a variety of growth factors that support retinal cells.

AIM

To test ASC neural differentiation using conditioned medium (CM) from an RPE cell line (ARPE-19).

METHODS

ASCs were isolated from adipose tissue, harvested from the subcutaneous region of healthy donors undergoing liposuction procedures. Four ASC culture conditions were investigated: ASCs cultured in basal Dulbecco's Modified Eagle Medium (DMEM); ASCs cultured in serum-free DMEM; ASCs cultured in serum-free DMEM/F12; and ASCs cultured in a CM from ARPE-19, a spontaneously arising cell line with a normal karyotype derived from a human RPE. Cell proliferation rate and viability were assessed by crystal violet and MTT assays at 1, 4 and 8 d of culture. At the same time points, ASC neural differentiation was evaluated by immunocytochemistry and western blot analysis for typical neuronal and glial markers: Nestin, neuronal specific enolase (NSE), protein gene product (PGP) 9.5, and glial fibrillary acidic protein (GFAP).

RESULTS

Depending on the culture medium, ASC proliferation rate and viability showed some significant differences. Overall, less dense populations were observed in serum-free cultures, except for ASCs cultured in ARPE-19 serum-free CM. Moreover, a different cell morphology was seen in these cultures after 8 d of treatment, with more elongated cells, often showing cytoplasmic ramifications. Immunofluorescence results and western blot analysis were indicative of ASC neural differentiation. In fact, basal levels of neural markers detected under control conditions significantly increased when cells were cultured in ARPE-19 CM. Specifically, neural marker overexpression was more marked at 8 d. The most evident increase was observed for NSE and GFAP, a modest increase was observed for nestin, and less relevant changes were observed for PGP9.5.

CONCLUSION

The presence of growth factors produced by ARPE-19 cells in tissue culture induces ASCs to express neural differentiation markers typical of the neuronal and glial cells of the retina.

Hydrogen sulfide: a gaseous signaling molecule modulates tissue homeostasis: implications in ophthalmic diseases

Hydrogen sulfide (H2S) serves as a gasotransmitter in the regulation of organ development and maintenance of homeostasis in tissues. Its abnormal levels are associated with multiple human diseases, such as neurodegenerative disease, myocardial injury, and ophthalmic diseases. Excessive exposure to H2S could lead to cellular toxicity, orchestrate pathological process, and increase the risk of various diseases. Interestingly, under physiological status, H2S plays a critical role in maintaining cellular physiology and limiting damages to tissues. In mammalian species, the generation of H2S is catalyzed by cystathionine beta-synthase (CBS), cystathionine gamma-lyase (CSE), 3-mercapto-methylthio pyruvate aminotransferase (3MST) and cysteine aminotransferase (CAT). These enzymes are found inside the mammalian eyeballs at different locations. Their aberrant expression and the accumulation of substrates and intermediates can change the level of H2S by orders of magnitude, causing abnormal structures or functions in the eyes. Detailed investigations have demonstrated that H2S donors' administration could regulate intraocular pressure, protect retinal cells, inhibit oxidative stress and alleviate inflammation by modulating the function of intra or extracellular proteins in ocular tissues. Thus, several slow-releasing H2S donors have been shown to be promising drugs for treating multiple diseases. In this review, we discuss the biological function of H2S metabolism and its application in ophthalmic diseases.

Induction of Neuronal Morphology in the 661W Cone Photoreceptor Cell Line with Staurosporine

PURPOSE

RGC-5 cells undergo differentiation into a neuronal phenotype with low concentrations of staurosporine. Although the RGC-5 cell line was initially thought to be of retinal ganglion cell origin, recent evidence suggests that the RGC-5 line could have been the result of contamination with 661W mouse cone photoreceptor cells. This raised the possibility that a cone photoreceptor cell line could be multipotent and could be differentiated to a neuronal phenotype.

METHODS

661W and RGC-5 cells, non-neuronal retinal astrocytes, retinal endothelial cells, retinal pericytes, M21 melanoma cells, K562 chronic myelogenous leukemia cells, and Daudi Burkitt lymphoma cells, were differentiated with staurosporine. The resulting morphology was quantitated using NeuronJ with respect to neurite counts and topology.

RESULTS

Treatment with staurosporine induced similar-appearing morphological differentiation in both 661W and RGC-5 cells. The following measures were not significantly different between 661W and RGC-5 cells: number of neurites per cell, total neurite field length, number of neurite branch points, and cell viability. Neuronal-like differentiation was not observed in the other cell lines tested.

CONCLUSIONS

661W and RGC-5 cells have virtually identical and distinctive morphology when differentiated with low concentrations of staurosporine. This result demonstrates that a retinal neuronal precursor cell with cone photoreceptor lineage can be differentiated to express a neuronal morphology.

Topographical control of ocular cell types for tissue engineering

Visual impairment affects over 285 million people worldwide and has a major impact on an individual's quality of life. Tissue engineering has the potential to increase the quality of life for many of these patients by preventing vision loss or restoring vision using cell-based therapies. However, these strategies will require an understanding of the microenvironmental factors that influence cell behavior. The eye is a well-organized organ whose structural complexity is essential for proper function. Interactions between ocular cells and their highly ordered extracellular matrix are necessary for maintaining key tissue properties including corneal transparency and retinal lamination. Therefore, it is not surprising that culturing these cells in vitro on traditional flat substrates result in irregular morphology. Instead, topographically patterned biomaterials better mimic native extracellular matrix and have been shown to elicit in vivo-like morphology and gene expression which is essential for tissue engineering. Herein we review multiple methods for producing well-controlled topography and discuss optimal biomaterial scaffold design for cells of the cornea, retina, and lens.

RPE-secreted factors: influence differentiation in human retinal cell line in dose- and density-dependent manner

Retinal pigment epithelial (RPE) cells play an important role in normal functioning of retina and photoreceptors, and some retinal degenerations arise due to malfunctioning RPE. Retinal pigment epithelium transplantation is being explored as a strategy to rescue degenerating photoreceptors in diseases such as age-related macular degeneration (AMD) and retinitis pigmentosa (RP). Additionally, RPE-secreted factors could rescue degenerating photoreceptors by prolonging survival or by their ability to differentiate and give rise to photoreceptors by transdifferentiation. In this study, we have explored what role cell density could play in differentiation induced in a human retinal progenitor cell line, in response to RPE-secreted growth factors. Retinal progenitors plated at low (1 × 10(4) cells/cm(2)), medium (2-4 × 10(4) cells/cm(2)), and high (1 × 10(5) cells/cm(2)) cell density were exposed to various dilutions of RPE-conditioned medium (secreted factors) under conditions of defined medium culture. Progenitor cell differentiation was monitored phenotypically (morphological, biochemical analysis, and immunophenotyping, and western blot analysis were performed). Our data show that differentiation in response to RPE-secreted factors is modulated by cell density and dilutions of conditioned medium. We conclude that before embarking on RPE transplantation as a modality for treatment of RP and AMD, one will have to determine the role that cell density and inhibitory and stimulatory neurotrophins secreted by RPE could play in the efficacy of survival of transplants. We report that RPE-conditioned medium enhances neuronal phenotype (photoreceptors, bipolars) at the lowest cell density in the absence of cell-cell contact. Eighty percent to 90% of progenitor cells differentiate into photoreceptors and bipolars at 50% concentration of conditioned medium, while exposure to 100% conditioned medium might increase multipolar neurons (ganglionic and amacrine phenotypes) to a small degree. However, no clear-cut pattern of differentiation in response to RPE-secreted factors is noted at higher cell densities.

Mesenchymal stem cells for retinal diseases

Retinal diseases are featured with the common result of retinal cell apoptosis that will cause irreversible vision loss. Various attempts have been made for the solution against cell death. However, few approaches turn out to be effective. With the progress in mesenchymal stem cells (MSCs) research, MSCs were considered as a promising source for cell replacement or neuroprotection in retinal disorders. MSCs have the property of self-renewal and are multipotent cells derived from various mesenchymal tissues, which were demonstrated being capable of differentiating into multilineage tissue cells. Some works were also done to differentiate MSCs into retinal cells. MSCs could be induced to express retinal cell markers under certain stimuli. Recent studies also suggest that MSCs should be an ideal source for neuroprotection via the secretion of a variety of neurotrophins. Engineered MSCs were also used as vehicles for continuous delivery of neurotrophins against retinal degeneration with encouraging results. Since there are still barriers on the differentiation of MSCs into functional retinal cells, the use of MSCs for neuroprotection in retinal diseases seems to be a more practicable approach and worthy of further investigations.

Regulation of prenatal human retinal neurosphere growth and cell fate potential by retinal pigment epithelium and Mash1

During development of the central nervous system, stem and progenitor cell proliferation and differentiation are controlled by complex inter- and intracellular interactions that orchestrate the precise spatiotemporal production of particular cell types. Within the embryonic retina, progenitor cells are located adjacent to the retinal pigment epithelium (RPE), which differentiates prior to the neurosensory retina and has the capacity to secrete a multitude of growth factors. We found that secreted proteinaceous factors in human prenatal RPE conditioned medium (RPE CM) prolonged and enhanced the growth of human prenatal retinal neurospheres. The growth-promoting activity of RPE CM was mitogen-dependent and associated with an acute increase in transcription factor phosphorylation. Expanded populations of RPE CM-treated retinal neurospheres expressed numerous neurodevelopmental and eye specification genes and markers characteristic of neural and retinal progenitor cells, but gradually lost the potential to generate neurons upon differentiation. Misexpression of Mash1 restored the neurogenic potential of long-term cultures, yielding neurons with phenotypic characteristics of multiple inner retinal cell types. Thus, a novel combination of extrinsic and intrinsic factors was required to promote both progenitor cell proliferation and neuronal multipotency in human retinal neurosphere cultures. These results support a pro-proliferative and antiapoptotic role for RPE in human retinal development, reveal potential limitations of human retinal progenitor culture systems, and suggest a means for overcoming cell fate restriction in vitro.