Photoreceptor vitality in organotypic cultures of mature vertebrate retinas validated by light-dependent molecular movements

Induction of Human Umbilical Mesenchymal Stem Cell Differentiation Into Retinal Pigment Epithelial Cells Using a Transwell-Based Co-culture System

There is an increasing interest in generating retinal pigment epithelial (RPE) cells from stem cells for treating degenerative eye diseases. However, whether human umbilical cord mesenchymal stem cells (HUCMSCs) can differentiate into RPE-like cells in a co-culture system has not been fully understood. In this study, induction of HUCMSC differentiation into RPE-like cells was performed by co-culturing HUCMSCs and a human RPE-like cell line (ARPE19) in a transwell system and then analyzed for biomarkers using quantitative reverse transcription polymerase chain reaction (RT-PCR) and immunofluorescence staining technique. Moreover, the functional characterization of induced cells was carried out by examining their phagocytic and neurotrophic factor–secreting activities. Our results showed that mRNA expressions of RPE-specific markers—MITF, OTX2, RPE65, PEDF, PME17, and CRALBP—and protein markers—RPE65, CRALBP, and ZO-1—were significantly increased in HUCMSC-derived RPE-like cells. Functional characteristic studies showed that these induced cells were capable of engulfing photoreceptor outer segments and secreting brain-derived neurotrophic factor (BDNF) and glial-derived neurotrophic factor (GDNF), which are typical functions of RPE-like cells. Overall, the study findings indicate that the morphology and proliferation of HUCMSCs can be maintained in a serum-free medium, and differentiation into RPE-like cells can be induced by simply co-culturing HUCMSCs with ARPE19 cells. Thus, the study provides fundamental information regarding the clinical-scale generation of RPE-like cells from HUCMSCs.

New Method for Efficient siRNA Delivery in Retina Explants: Reverse Magnetofection

The prevalence of retinal disorders associated with visual impairment and blindness is increasing worldwide, while most of them remain without effective treatment. Pharmacological and molecular therapy development is hampered by the lack of effective drug delivery into the posterior segment of the eye. Among molecular approaches, RNA-interference (RNAi) features strong advantages, yet delivering it to the inner layer of the retina appears extremely challenging. To address this, we developed an original magnetic nanoparticles (MNPs)-based transfection method that allows the efficient delivery of siRNA in all retinal layers of rat adult retinas through magnetic targeting. To establish delivery of RNAi throughout the retina, we have chosen organotypic retinal explants as an ex vivo model and for future high content screening of molecular drugs. Conversely to classic Magnetofection, and similar to conditions in the posterior chamber of the eye, our methods allows attraction of siRNA complexed to MNPs from the culture media into the explant. Our method termed "Reverse Magnetofection" provides a novel and nontoxic strategy for RNAi-based molecular as well as gene therapy in the retina that can be transferred to a wide variety of organ explants.

Tissue engineering of retina through high resolution 3-dimensional inkjet bioprinting

The mammalian retina contains multiple cellular layers, each carrying out a specific task. Such a controlled organization should be considered as a crucial factor for designing retinal therapies. The maintenance of retinal layered complexity through the use of scaffold-free techniques has recently emerged as a promising approach for clinical ocular tissue engineering. In an attempt to fabricate such layered retinal model, we are proposing herein a unique inkjet bioprinting system applied to the deposition of a photoreceptor cells (PRs) layer on top of a bioprinted retinal pigment epithelium (RPE), in a precise arrangement and without any carrier material. The results showed that, after bioprinting, both RPE and PRs were well positioned in a layered structure and expressed their structural markers, which was further demonstrated by ZO1, MITF, rhodopsin, opsin B, opsin R/G and PNA immunostaining, three days after bioprinting. We also showed that considerable amounts of human vascular endothelial growth factors (hVEGF) were released from the RPE printed layer, which confirmed the formation of a functional RPE monolayer after bioprinting. Microstructures of bioprinted cells as well as phagocytosis of photoreceptor outer segments by apical RPE microvilli were finally established through transmission electron microscopy (TEM) imaging. In summary, using this carrier-free bioprinting method, it was possible to develop a reasonable in vitro retina model for studying some sight-threatening diseases, such as age-related macular degeneration (AMD) and retinitis pigmentosa (RP).

The use of in vivo, ex vivo, in vitro, computational models and volunteer studies in vision research and therapy, and their contribution to the Three Rs

Much is known about mammalian vision, and considerable progress has been achieved in treating many vision disorders, especially those due to changes in the eye, by using various therapeutic methods, including stem cell and gene therapy. While cells and tissues from the main parts of the eye and the visual cortex (VC) can be maintained in culture, and many computer models exist, the current non-animal approaches are severely limiting in the study of visual perception and retinotopic imaging. Some of the early studies with cats and non-human primates (NHPs) are controversial for animal welfare reasons and are of questionable clinical relevance, particularly with respect to the treatment of amblyopia. More recently, the UK Home Office records have shown that attention is now more focused on rodents, especially the mouse. This is likely to be due to the perceived need for genetically-altered animals, rather than to knowledge of the similarities and differences of vision in cats, NHPs and rodents, and the fact that the same techniques can be used for all of the species. We discuss the advantages and limitations of animal and non-animal methods for vision research, and assess their relative contributions to basic knowledge and clinical practice, as well as outlining the opportunities they offer for implementing the principles of the Three Rs (Replacement, Reduction and Refinement).

Protein sorting, targeting and trafficking in photoreceptor cells

Vision is the most fundamental of our senses initiated when photons are absorbed by the rod and cone photoreceptor neurons of the retina. At the distal end of each photoreceptor resides a light-sensing organelle, called the outer segment, which is a modified primary cilium highly enriched with proteins involved in visual signal transduction. At the proximal end, each photoreceptor has a synaptic terminal, which connects this cell to the downstream neurons for further processing of the visual information. Understanding the mechanisms involved in creating and maintaining functional compartmentalization of photoreceptor cells remains among the most fascinating topics in ocular cell biology. This review will discuss how photoreceptor compartmentalization is supported by protein sorting, targeting and trafficking, with an emphasis on the best-studied cases of outer segment-resident proteins.

A comparative evaluation of NB30, NB54 and PTC124 in translational read-through efficacy for treatment of an USH1C nonsense mutation

Translational read-through-inducing drugs (TRIDs) promote read-through of nonsense mutations, placing them in the spotlight of current gene-based therapeutic research. Here, we compare for the first time the relative efficacies of new-generation aminoglycosides NB30, NB54 and the chemical compound PTC124 on retinal toxicity and read-through efficacy of a nonsense mutation in the USH1C gene, which encodes the scaffold protein harmonin. This mutation causes the human Usher syndrome, the most common form of inherited deaf-blindness. We quantify read-through efficacy of the TRIDs in cell culture and show the restoration of harmonin function. We do not observe significant differences in the read-through efficacy of the TRIDs in retinal cultures; however, we show an excellent biocompatibility in retinal cultures with read-through versus toxicity evidently superior for NB54 and PTC124. In addition, in vivo administration of NB54 and PTC124 induced recovery of the full-length harmonin a1 with the same efficacy. The high biocompatibilities combined with the sustained read-through efficacies of these drugs emphasize the potential of NB54 and PTC124 in treating nonsense mutation-based retinal disorders.

Pharmacological modulation of the retinal unfolded protein response in Bardet-Biedl syndrome reduces apoptosis and preserves light detection ability

Ciliopathies, a class of rare genetic disorders, present often with retinal degeneration caused by protein transport defects between the inner segment and the outer segment of the photoreceptors. Bardet-Biedl syndrome is one such ciliopathy, genetically heterogeneous with 17 BBS genes identified to date, presenting early onset retinitis pigmentosa. By investigating BBS12-deprived retinal explants and the Bbs12(-/-) murine model, we show that the impaired intraciliary transport results in protein retention in the endoplasmic reticulum. The protein overload activates a proapoptotic unfolded protein response leading to a specific Caspase12-mediated death of the photoreceptors. Having identified a therapeutic window in the early postnatal retinal development and through optimized pharmacological modulation of the unfolded protein response, combining three specific compounds, namely valproic acid, guanabenz, and a specific Caspase12 inhibitor, achieved efficient photoreceptor protection, thereby maintaining light detection ability in vivo.

Tailoring substrates for long-term organotypic culture of adult neuronal tissue

Organotypic tissue cultures are highly promising for performing in vivo type studies in vitro. Currently, however, very limited survival times of only a few days for adult tissue often severely limit their application. Here, superhydrophilic nanostructured substrates with ideal material properties ensure tissue adhesion, essential for organotypic culture, while migration of single cells out of the tissue is hampered. Tuning substrate properties, for the first time, adult neuronal tissue could be cultured for 14 days with no indications of degeneration.

Photoreceptor signaling: supporting vision across a wide range of light intensities

For decades, photoreceptors have been an outstanding model system for elucidating basic principles in sensory transduction and biochemistry and for understanding many facets of neuronal cell biology. In recent years, new knowledge of the kinetics of signaling and the large-scale movements of proteins underlying signaling has led to a deeper appreciation of the photoreceptor's unique challenge in mediating the first steps in vision over a wide range of light intensities.

A component of retinal light adaptation mediated by the thyroid hormone cascade

Analysis with DNA-microrrays and real time PCR show that several genes involved in the thyroid hormone cascade, such as deiodinase 2 and 3 (Dio2 and Dio3) are differentially regulated by the circadian clock and by changes of the ambient light. The expression level of Dio2 in adult rats (2-3 months of age) kept continuously in darkness is modulated by the circadian clock and is up-regulated by 2 fold at midday. When the diurnal ambient light was on, the expression level of Dio2 increased by 4-8 fold and a consequent increase of the related protein was detected around the nuclei of retinal photoreceptors and of neurons in inner and outer nuclear layers. The expression level of Dio3 had a different temporal pattern and was down-regulated by diurnal light. Our results suggest that DIO2 and DIO3 have a role not only in the developing retina but also in the adult retina and are powerfully regulated by light. As the thyroid hormone is a ligand-inducible transcription factor controlling the expression of several target genes, the transcriptional activation of Dio2 could be a novel genomic component of light adaptation.

PTC124-mediated translational readthrough of a nonsense mutation causing Usher syndrome type 1C

We investigated the therapeutic potential of the premature termination codon (PTC) readthrough-inducing drug PTC124 in treating the retinal phenotype of Usher syndrome, caused by a nonsense mutation in the USH1C gene. Applications in cell culture, organotypic retina cultures, and mice in vivo revealed significant readthrough and the recovery of protein function. In comparison with other readthrough drugs, namely the clinically approved readthrough-inducing aminoglycoside gentamicin, PTC124 exhibits significant better retinal biocompatibility. Its high readthrough efficiency in combination with excellent biocompatibility makes PTC124 a promising therapeutic agent for PTCs in USH1C, as well as other ocular and nonocular genetic diseases.

Light-dependent translocation of arrestin in rod photoreceptors is signaled through a phospholipase C cascade and requires ATP

Partitioning of cellular components is a critical mechanism by which cells can regulate their activity. In rod photoreceptors, light induces a large-scale translocation of arrestin from the inner segments to the outer segments. The purpose of this project is to elucidate the signaling pathway necessary to initiate arrestin translocation to the outer segments and the mechanism for arrestin translocation. Mouse retinal organotypic cultures and eyes from transgenic Xenopus tadpoles expressing a fusion of GFP and rod arrestin were treated with both activators and inhibitors of proteins in the phosphoinositide pathway. Confocal microscopy was used to image the effects of the pharmacological agents on arrestin translocation in rod photoreceptors. Retinas were also depleted of ATP using potassium cyanide to assess the requirement for ATP in arrestin translocation. In this study, we demonstrate that components of the G-protein-linked phospholipase C (PLC) pathway play a role in initiating arrestin translocation. Our results show that arrestin translocation can be stimulated by activators of PLC and protein kinase C (PKC), and by cholera toxin in the absence of light. Arrestin translocation to the outer segments is significantly reduced by inhibitors of PLC and PKC. Importantly, we find that treatment with potassium cyanide inhibits arrestin translocation in response to light. Collectively, our results suggest that arrestin translocation is initiated by a G-protein-coupled cascade through PLC and PKC signaling. Furthermore, our results demonstrate that at least the initiation of arrestin translocation requires energy input.

Prolonged illumination up-regulates arrestin and two guanylate cyclase activating proteins: a novel mechanism for light adaptation

Light adaptation in vertebrate photoreceptors is mediated by multiple mechanisms, one of which could involve nuclear feedback and changes in gene expression. Therefore, we have investigated light adaptation-associated changes in gene expression using microarrays and real-time PCR in isolated photoreceptors, in cultured isolated retinas and in acutely isolated retinas. In all three preparations after 2 h of an exposure to a bright light, we observed an up-regulation of almost 100% of three genes, Sag, Guca1a and Guca1b, coding for proteins known to play a major role in phototransduction: arrestin, GCAP1 and GCAP2. No detectable up-regulation occurred for light exposures of less than 1 h. Functional in vivo electroretinographic tests show that a partial recovery of the dark current occurred 1-2 h after prolonged illumination with a steady light that initially caused a substantial suppression of the photoresponse. These observations demonstrate that prolonged illumination results in the up-regulation of genes coding for proteins involved in the phototransduction signalling cascade, possibly underlying a novel component of light adaptation occurring 1-2 h after the onset of a steady bright light.

The translocation of signaling molecules in dark adapting mammalian rod photoreceptor cells is dependent on the cytoskeleton

In vertebrate rod photoreceptor cells, arrestin and the visual G-protein transducin move between the inner segment and outer segment in response to changes in light. This stimulus dependent translocation of signalling molecules is assumed to participate in long term light adaptation of photoreceptors. So far the cellular basis for the transport mechanisms underlying these intracellular movements remains largely elusive. Here we investigated the dependency of these movements on actin filaments and the microtubule cytoskeleton of photoreceptor cells. Co-cultures of mouse retina and retinal pigment epithelium were incubated with drugs stabilizing and destabilizing the cytoskeleton. The actin and microtubule cytoskeleton and the light dependent distribution of signaling molecules were subsequently analyzed by light and electron microscopy. The application of cytoskeletal drugs differentially affected the cytoskeleton in photoreceptor compartments. During dark adaptation the depolymerization of microtubules as well as actin filaments disrupted the translocation of arrestin and transducin in rod photoreceptor cells. During light adaptation only the delivery of arrestin within the outer segment was impaired after destabilization of microtubules. Movements of transducin and arrestin required intact cytoskeletal elements in dark adapting cells. However, diffusion might be sufficient for the fast molecular movements observed as cells adapt to light. These findings indicate that different molecular translocation mechanisms are responsible for the dark and light associated translocations of arrestin and transducin in rod photoreceptor cells.

Murine cytomegalovirus infection and apoptosis in organotypic retinal cultures

PURPOSE

An organotypic retinal culture model was used to determine the pattern of murine cytomegalovirus (MCMV) infection and whether apoptosis is induced in MCMV-infected cultured retinas.

METHODS

Retinas harvested from C57BL/6 mice were individually cultured at 37 degrees C on 3-microm filter inserts placed in 24-well plates. Some retinas were infected with MCMV (5 x 10(5) PFU/well). At days 4, 7, and 11 after infection (pi), the culture medium and cultured retinas were collected for examination.

RESULTS

Replicating virus was recovered and viral early antigen (EA)- and late antigen (LA)-positive cells were observed in the MCMV-infected retinal cultures. Most MCMV-infected cells were glia and horizontal cells. Infection resulted in atrophy of the photoreceptor cells and cytomegaly. Apoptosis of uninfected bystander cells, including photoreceptor cells and horizontal cells, was observed. TNF-alpha was produced by activated microglia during MCMV infection of the retina. Mouse apoptosis microarray studies, caspase activity studies, and RT-PCR studies showed that the genes involved in both the death receptor-mediated apoptotic pathway and the mitochondrial pathway were upregulated.

CONCLUSIONS

Many aspects of MCMV infection of retinal cultures parallel those observed during MCMV retinitis in mice. Thus, this in vitro system may be used to explore the role of apoptosis of uninfected retinal cells and the contribution of cytokines and other modulators to the pathogenesis of CMV retinitis.

A novel Usher protein network at the periciliary reloading point between molecular transport machineries in vertebrate photoreceptor cells

The human Usher syndrome (USH) is the most frequent cause of combined deaf-blindness. USH is genetically heterogeneous with at least 12 chromosomal loci assigned to three clinical types, USH1-3. Although these USH types exhibit similar phenotypes in human, the corresponding gene products belong to very different protein classes and families. The scaffold protein harmonin (USH1C) was shown to integrate all identified USH1 and USH2 molecules into protein networks. Here, we analyzed a protein network organized in the absence of harmonin by the scaffold proteins SANS (USH1G) and whirlin (USH2D). Immunoelectron microscopic analyses disclosed the colocalization of all network components in the apical inner segment collar and the ciliary apparatus of mammalian photoreceptor cells. In this complex, whirlin and SANS directly interact. Furthermore, SANS provides a linkage to the microtubule transport machinery, whereas whirlin may anchor USH2A isoform b and VLGR1b (very large G-protein coupled receptor 1b) via binding to their cytodomains at specific membrane domains. The long ectodomains of both transmembrane proteins extend into the gap between the adjacent membranes of the connecting cilium and the apical inner segment. Analyses of Vlgr1/del7TM mice revealed the ectodomain of VLGR1b as a component of fibrous links present in this gap. Comparative analyses of mouse and Xenopus photoreceptors demonstrated that this USH protein network is also part of the periciliary ridge complex in Xenopus. Since this structural specialization in amphibian photoreceptor cells defines a specialized membrane domain for docking and fusion of transport vesicles, we suggest a prominent role of the USH proteins in cargo shipment.

Bioluminescent imaging of Ca2+ activity reveals spatiotemporal dynamics in glial networks of dark-adapted mouse retina

Glial Ca(2+) excitability plays a key role in reciprocal neuron-glia communication. In the retina, neuron-glia signalling is expected to be maximal in the dark, but the glial Ca(2+) signal characteristics under such conditions have not been evaluated. To address this question, we used bioluminescence imaging to monitor spontaneous Ca(2+) changes under dark conditions selectively in Müller cells, the principal retinal glial cells. By combining this imaging approach with network analysis, we demonstrate that activity in Müller cells is organized in networks of coactive cells, involving 2-16 cells located distantly and/or in clusters. We also report that spontaneous activity of small networks (2-6 Müller cells) repeat over time, sometimes in the same sequential order, revealing specific temporal dynamics. In addition, we show that networks of coactive glial cells are inhibited by TTX, indicating that ganglion and/or amacrine neuronal cells probably regulate Müller cell network properties. These results represent the first demonstration that spontaneous activity in adult Müller cells is patterned into correlated networks that display repeated sequences of coactivations over time. Furthermore, our bioluminescence technique provides a novel tool to study the dynamic characteristics of glial Ca(2+) events in the retina under dark conditions, which should greatly facilitate future investigations of retinal dark-adaptive processes.

Organotypic culture of physiologically functional adult mammalian retinas

Background

The adult mammalian retina is an important model in research on the central nervous system. Many experiments require the combined use of genetic manipulation, imaging, and electrophysiological recording, which make it desirable to use an in vitro preparation. Unfortunately, the tissue culture of the adult mammalian retina is difficult, mainly because of the high energy consumption of photoreceptors.

Methods and Findings

We describe an interphase culture system for adult mammalian retina that allows for the expression of genes delivered to retinal neurons by particle-mediated transfer. The retinas retain their morphology and function for up to six days— long enough for the expression of many genes of interest—so that effects upon responses to light and receptive fields could be measured by patch recording or multielectrode array recording. We show that a variety of genes encoding pre- and post-synaptic marker proteins are localized correctly in ganglion and amacrine cells.

Conclusions

In this system the effects on neuronal function of one or several introduced exogenous genes can be studied within intact neural circuitry of adult mammalian retina. This system is flexible enough to be compatible with genetic manipulation, imaging, cell transfection, pharmacological assay, and electrophysiological recordings.