Photoreceptor survival and development in culture

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

A photoreceptor cell line, designated 661W, was tested for its response to growth factors secreted by retinal pigment epithelial cells including basic fibroblast growth factor, epidermal growth factor, and nerve growth factor. Early passaged 661W cells expressed high levels of retinal progenitor markers such as nestin and Pax6, but not opsin or glial fibrillary acidic protein. 661W cells grown in FGF-2 or EGF exhibited a multiple-process morphology with small phase-bright nuclei similar to neurons, whereas cells cultured in nerve growth factor (NGF) or retinal pigment epithelium (RPE)-conditioned medium (RPE-CM) displayed rounded profiles lacking processes. 661W cells grown in FGF-2 were slightly elevated, but not significantly above, control cultures; but cells treated with RPE-CM or NGF were fewer, approximately 63% and 49% of control, respectively. NGF immunodepletion of RPE-CM strongly suppressed the inhibitory activity of RPE-CM on cell proliferation. Cells treated with FGF-2, but not NGF, upregulated their expression of opsin. All treatment conditions resulted in almost 100% viability based on calcium AM staining. Cells grown on extracellular matrix proteins laminin, fibronectin, and/or collagen resembled those grown on untreated dishes. This study showed that early passaged 661W cells displayed characteristics of retinal progenitor cells. The 661W cells proliferated and appeared to mature morphologically expressing rod photoreceptor phenotype in response to FGF-2. In contrast, NGF and RPE-CM inhibited proliferation and morphological differentiation of 661W cells, possibly inducing cell cycle arrest. These findings are consistent with reports that the RPE modulates photoreceptor differentiation and retinal progenitor cells via secreted factors and may play a role in the regulation of the retinal stem cell niche.

Mouse retina explants after long-term culture in serum free medium

The neonatal mouse retina remains viable as an explant in serum-supplemented growth media for more than 4 weeks. Interpretation of drug effects on this tissue is compromised by the enigmatic composition of the serum. We sought to remove this ambiguity by culturing neonatal as well as late postnatal mouse retina in serum-free nutrient medium. In this study three important observations were made, (1) there is histotypic development of neonatal as well as preservation of late postnatal mouse retinal structure during long-term culture in serum-free medium, although the late postnatal tissue tends to show some loss of cells in the outer nuclear layer. (2) Protein expression in explant photoreceptor cells was similar to that in the litter-matched ones, except for green cone opsin and interphotoreceptor retinoid-binding protein, although mRNA of the latter is present at similar amounts as in age-matched in vivo controls. (3) Cells of the inner retina stained by antibodies to calcium-binding proteins display some novel sprouting of processes. The results show that the mouse retina can be cultured as an explant for more than 4 weeks in a serum-free medium. This represents an important step forward because, (1) the possibility of interference of drug effects by unknown serum factors has been eliminated; and (2) the spent culture medium can be analyzed to investigate biomolecules released by the retina in vitro.

Renewal of photoreceptor outer segments and their phagocytosis by the retinal pigment epithelium

The discovery of disc protein renewal in rod outer segments, in 1960s, was followed by the observation that old discs were ingested by the retinal pigment epithelium. This process occurs in both rods and cones and is crucial for their survival. Photoreceptors completely degenerate in the Royal College of Surgeons mutant rat, whose pigment epithelium cannot ingest old discs. The complete renewal process includes the following sequential steps involving both photoreceptor and pigment epithelium activity: new disc assembly and old disc shedding by photoreceptor cells; recognition and binding to pigment epithelium membranes; then ingestion, digestion, and segregation of residual bodies in pigment epithelium cytoplasm. Regulating factors are involved at each step. While disc assembly is mostly genetically controlled, disc shedding and the subsequent pigment epithelium phagocytosis appear regulated by environmental factors (light and temperature). Disc shedding is rhythmically controlled by an eye intrinsic circadian oscillator using endogenous dopamine and melatonin as light and dark signal, respectively. Of special interest is the regulation of phagocytosis by multiple receptors, including specific phagocytosis receptors and receptors for neuroactive substances released from the neuroretina. The candidates for phagocytosis receptors are presented, but it is acknowledged that they are not completely known. The main neuromodulators are adenosine, dopamine, glutamate, serotonin, and melatonin. Although the transduction mechanisms are not fully understood, attention was brought to cyclic AMP, phosphoinositides, and calcium. The chapter points to the multiplicity of regulating factors and the complexity of their intermingling modes of action. Promising areas for future research still exist in this field.

A reliable method for organ culture of neonatal mouse retina with long-term survival

Organ culture systems of the central nervous system have proven to be useful tools for the study of development, differentiation, and degeneration. Some studies have been limited by the inability to maintain the cultures over an extended period. Here we describe an organ culture technique for the mouse retina. This method uses commercially available supplies and reproducible procedures to maintain healthy retinas with normal architecture for 4 weeks in vitro. The system is amenable to quantitative analysis. It can be used with both normal and retinal degeneration (rd) retinas to study of the role of various factors in photoreceptor degeneration in retinal cell fate determination and development.