Histogenesis of the avian retina in reaggregation culture: from dissociated cells to laminar neuronal networks

Pluripotent stem cell-derived models of retinal disease: Elucidating pathogenesis, evaluating novel treatments, and estimating toxicity

Blindness poses a growing global challenge, with approximately 26% of cases attributed to degenerative retinal diseases. While gene therapy, optogenetic tools, photosensitive switches, and retinal prostheses offer hope for vision restoration, these high-cost therapies will benefit few patients. Understanding retinal diseases is therefore key to advance effective treatments, requiring in vitro models replicating pathology and allowing quantitative assessments for drug discovery. Pluripotent stem cells (PSCs) provide a unique solution given their limitless supply and ability to differentiate into light-responsive retinal tissues encompassing all cell types. This review focuses on the history and current state of photoreceptor and retinal pigment epithelium (RPE) cell generation from PSCs. We explore the applications of this technology in disease modelling, experimental therapy testing, biomarker identification, and toxicity studies. We consider challenges in scalability, standardisation, and reproducibility, and stress the importance of incorporating vasculature and immune cells into retinal organoids. We advocate for high-throughput automation in data acquisition and analyses and underscore the value of advanced micro-physiological systems that fully capture the interactions between the neural retina, RPE, and choriocapillaris.

Self-Organization of the Retina during Eye Development, Retinal Regeneration In Vivo, and in Retinal 3D Organoids In Vitro

Self-organization is a process that ensures histogenesis of the eye retina. This highly intricate phenomenon is not sufficiently studied due to its biological complexity and genetic heterogeneity. The review aims to summarize the existing central theories and ideas for a better understanding of retinal self-organization, as well as to address various practical problems of retinal biomedicine. The phenomenon of self-organization is discussed in the spatiotemporal context and illustrated by key findings during vertebrate retina development in vivo and retinal regeneration in amphibians in situ. Described also are histotypic 3D structures obtained from the disaggregated retinal progenitor cells of birds and retinal 3D organoids derived from the mouse and human pluripotent stem cells. The review highlights integral parts of retinal development in these conditions. On the cellular level, these include competence, differentiation, proliferation, apoptosis, cooperative movements, and migration. On the physical level, the focus is on the mechanical properties of cell- and cell layer-derived forces and on the molecular level on factors responsible for gene regulation, such as transcription factors, signaling molecules, and epigenetic changes. Finally, the self-organization phenomenon is discussed as a basis for the production of retinal organoids, a promising model for a wide range of basic scientific and medical applications.

Potential Endogenous Cell Sources for Retinal Regeneration in Vertebrates and Humans: Progenitor Traits and Specialization

Retinal diseases often cause the loss of photoreceptor cells and, consequently, impairment of vision. To date, several cell populations are known as potential endogenous retinal regeneration cell sources (RRCSs): the eye ciliary zone, the retinal pigment epithelium, the iris, and Müller glia. Factors that can activate the regenerative responses of RRCSs are currently under investigation. The present review considers accumulated data on the relationship between the progenitor properties of RRCSs and the features determining their differentiation. Specialized RRCSs (all except the ciliary zone in low vertebrates), despite their differences, appear to be partially "prepared" to exhibit their plasticity and be reprogrammed into retinal neurons due to the specific gene expression and epigenetic landscape. The "developmental" characteristics of RRCS gene expression are predefined by the pathway by which these cell populations form during eye morphogenesis; the epigenetic features responsible for chromatin organization in RRCSs are under intracellular regulation. Such genetic and epigenetic readiness is manifested in vivo in lower vertebrates and in vitro in higher ones under conditions permissive for cell phenotype transformation. Current studies on gene expression in RRCSs and changes in their epigenetic landscape help find experimental approaches to replacing dead cells through recruiting cells from endogenous resources in vertebrates and humans.

Disaggregation and Reaggregation of Zebrafish Retinal Cells for the Analysis of Neuronal Layering

The reaggregation of dissociated cells to form organotypic structures provides an in vitro system for the analysis of the cellular interactions and molecular mechanisms involved in the formation of tissue architecture. The retina, an outgrowth of the forebrain, is a precisely layered neural tissue, yet the mechanisms underlying layer formation are largely unexplored. Here we describe the protocol to dissociate, re-aggregate, and culture zebrafish retinal cells from a transgenic, Spectrum of Fates, line where all main cell types are labelled with a combination of fluorescent proteins driven by fate-specific promoters. These cells re-aggregate and self-organize in just 48 h in minimal culture conditions. We also describe how the patterning in these aggregates can be analyzed using isocontour profiling to compare whether different conditions affect their self-organization.

Neuronal Migration and Lamination in the Vertebrate Retina

In the retina, like in most other brain regions, developing neurons are arranged into distinct layers giving the mature tissue its stratified appearance. This process needs to be highly controlled and orchestrated, as neuronal layering defects lead to impaired retinal function. To achieve successful neuronal layering and lamination in the retina and beyond, three main developmental steps need to be executed: First, the correct type of neuron has to be generated at a precise developmental time. Second, as most retinal neurons are born away from the position at which they later function, newborn neurons have to move to their final layer within the developing tissue, a process also termed neuronal lamination. Third, these neurons need to connect to their correct synaptic partners. Here, we discuss neuronal migration and lamination in the vertebrate retina and summarize our knowledge on these aspects of retinal development. We give an overview of how lamination emerges and discuss the different modes of neuronal translocation that occur during retinogenesis and what we know about the cell biological machineries driving them. In addition, retinal mosaics and their importance for correct retinal function are examined. We close by stating the open questions and future directions in this exciting field.

Self-organising aggregates of zebrafish retinal cells for investigating mechanisms of neural lamination

To investigate the cell-cell interactions necessary for the formation of retinal layers, we cultured dissociated zebrafish retinal progenitors in agarose microwells. Within these wells, the cells re-aggregated within hours, forming tight retinal organoids. Using a Spectrum of Fates zebrafish line, in which all different types of retinal neurons show distinct fluorescent spectra, we found that by 48 h in culture, the retinal organoids acquire a distinct spatial organisation, i.e. they became coarsely but clearly laminated. Retinal pigment epithelium cells were in the centre, photoreceptors and bipolar cells were next most central and amacrine cells and retinal ganglion cells were on the outside. Image analysis allowed us to derive quantitative measures of lamination, which we then used to find that Müller glia, but not RPE cells, are essential for this process.

Müller glia: Stem cells for generation and regeneration of retinal neurons in teleost fish

Adult zebrafish generate new neurons in the brain and retina throughout life. Growth-related neurogenesis allows a vigorous regenerative response to damage, and fish can regenerate retinal neurons, including photoreceptors, and restore functional vision following photic, chemical, or mechanical destruction of the retina. Müller glial cells in fish function as radial-glial-like neural stem cells. During adult growth, Müller glial nuclei undergo sporadic, asymmetric, self-renewing mitotic divisions in the inner nuclear layer to generate a rod progenitor that migrates along the radial fiber of the Müller glia into the outer nuclear layer, proliferates, and differentiates exclusively into rod photoreceptors. When retinal neurons are destroyed, Müller glia in the immediate vicinity of the damage partially and transiently dedifferentiate, re-express retinal progenitor and stem cell markers, re-enter the cell cycle, undergo interkinetic nuclear migration (characteristic of neuroepithelial cells), and divide once in an asymmetric, self-renewing division to generate a retinal progenitor. This daughter cell proliferates rapidly to form a compact neurogenic cluster surrounding the Müller glia; these multipotent retinal progenitors then migrate along the radial fiber to the appropriate lamina to replace missing retinal neurons. Some aspects of the injury-response in fish Müller glia resemble gliosis as observed in mammals, and mammalian Müller glia exhibit some neurogenic properties, indicative of a latent ability to regenerate retinal neurons. Understanding the specific properties of fish Müller glia that facilitate their robust capacity to generate retinal neurons will inform and inspire new clinical approaches for treating blindness and visual loss with regenerative medicine.

A novel automated spike sorting algorithm with adaptable feature extraction

To study the electrophysiological properties of neuronal networks, in vitro studies based on microelectrode arrays have become a viable tool for analysis. Although in constant progress, a challenging task still remains in this area: the development of an efficient spike sorting algorithm that allows an accurate signal analysis at the single-cell level. Most sorting algorithms currently available only extract a specific feature type, such as the principal components or Wavelet coefficients of the measured spike signals in order to separate different spike shapes generated by different neurons. However, due to the great variety in the obtained spike shapes, the derivation of an optimal feature set is still a very complex issue that current algorithms struggle with. To address this problem, we propose a novel algorithm that (i) extracts a variety of geometric, Wavelet and principal component-based features and (ii) automatically derives a feature subset, most suitable for sorting an individual set of spike signals. Thus, there is a new approach that evaluates the probability distribution of the obtained spike features and consequently determines the candidates most suitable for the actual spike sorting. These candidates can be formed into an individually adjusted set of spike features, allowing a separation of the various shapes present in the obtained neuronal signal by a subsequent expectation maximisation clustering algorithm. Test results with simulated data files and data obtained from chick embryonic neurons cultured on microelectrode arrays showed an excellent classification result, indicating the superior performance of the described algorithm approach.

Electromagnetic exposure of scaffold-free three-dimensional cell culture systems

In recent years, a number of in vitro studies have reported on the possible athermal effects of electromagnetic exposure on biological tissue. Typically, this kind of study is performed on monolayers of primary cells or cell lines. However, two-dimensional cell layer systems lack physiological relevance since cells in vivo are organized in a three-dimensional (3D) architecture. In monolayer studies, cell-cell and cell-ECM interactions obviously differ from live tissue and scale-ups of experimental results to in vivo systems should be considered carefully. To overcome this problem, we used a scaffold-free 3D cell culture system, suitable for the exploration of electrophysiological effects due to electromagnetic fields (EMF) at 900 MHz. Dissociated cardiac myocytes were reaggregated into cellular spheres by constant rotation, and non-invasive extracellular recordings of these so-called spheroids were performed with microelectrode arrays (MEA). In this study, 3D cell culture systems were exposed to pulsed EMFs in a stripline setup. We found that inhomogeneities in the EMF due to electrodes and conducting lines of the MEA chip had only a minor influence on the field distribution in the spheroid if the exposure parameters were chosen carefully.

Nuclei of chicken neurons in tissues and three-dimensional cell cultures are organized into distinct radial zones

We used chicken retinospheroids (RS) to study the nuclear architecture of vertebrate cells in a three-dimensional (3D) cell culture system. The results showed that the different neuronal cell types of RS displayed an extreme form of radial nuclear organization. Chromatin was arranged into distinct radial zones which became already visible after DAPI staining. The distinct zones were enriched in different chromatin modifications and in different types of chromosomes. Active isoforms of RNA polymerase II were depleted in the outermost zone. Also chromocenters and nucleoli were radially aligned in the nuclear interior. The splicing factor SC35 was enriched at the central zone and did not show the typical speckled pattern of distribution. Evaluation of neuronal and non-neuronal chicken tissues showed that the highly ordered form of radial nuclear organization was also present in neuronal chicken tissues. Furthermore, the data revealed that the neuron-specific nuclear organization was remodeled when cells spread on a flat substrate. Monolayer cultures of a chicken cell line did not show this extreme form of radial organization. Rather, such monolayer cultures displayed features of nuclear organization which have been described before for many different types of monolayer cells. The finding that an extreme form radial nuclear organization, which has not been described before, is present in RS and tissues, but not in cells spread on a flat substrate, suggests that it would be important to complement studies on nuclear architecture performed with monolayer cells by studies on 3D cell culture systems and tissues.

Roller Culture of Free-Floating Retinal Slices: A New System of Organotypic Cultures of Adult Rat Retina

No experimental system exists to date for the in vitro study of retinal ganglion cell populations in a three-dimensional organotypic tissue environment. Here, we describe such a novel method for roller cultivation of adult retinas. Retinas of adult (1-3 months old) rats were cut into rectangular slices of approximately 1 mm(2). Free-floating slices were cultured on a horizontal rotating roller drum (50-60 rpm) in a dry incubator at 36.5 degrees C. During the first days of cultivation, primary flat retinal slices changed their configuration and transformed into ball-shaped tissue spheres (retinal bodies). Histological and immunocytochemical studies showed that the outer wall of the retinal bodies was formed by cell and fibre layers typical of mature retina with photoreceptors located on the outside. Initially, retinal bodies contained an inner cavity which later was completely obliterated and filled with glial cells, sprouting nerve fibres, and vascular structures. This culture system was further developed into a robust model of glutamate-induced neurotoxicity. Using a novel culture method of adult rat retina, preservation of the three-dimensional organotypic retinal cytoarchitecture was achieved, including survival of neurons in the ganglion cell layer and sprouting of nerve fibres of the axotomized retinal ganglion cells. This novel culture model promises to facilitate studies of retinal physiology and pathology.

Exposure to Diazinon Alters In Vitro Retinogenesis: Retinospheroid Morphology, Development of Chicken Retinal Cell Types, and Gene Expression

Developing embryos are more vulnerable than adults to acute cholinergic intoxication by anticholinesterases, including organophosphorus pesticides. These agents affect the process of neural development itself, leading to permanent deficits in the architecture of the nervous system. Recent evidence on direct roles of acetylcholinesterase (AChE) on neuronal differentiation provides additional grounds for investigating the developmental toxicity of anticholinesterases. Therefore, the effect of the organophosphate diazinon on the development of chick retinal differentiation was studied by an in vitro reaggregate approach. Reaggregated spheres from dissociated retinal cells of the E6 chick embryo were produced in rotation culture. During the whole culture period of 10 days, experimental cultures were supplemented with different concentrations of the pesticide, from 20 to 120 microM diazinon. The pesticide-treated spheres were reduced in size, and their outer surface was irregular. More importantly, inner structural distortions could be easily traced because the structure of control spheroids can be well characterized by a histotypical arrangement of laminar parts homologous to the normal retina. Acetylcholinesterase activity in diazinon-treated spheres was reduced when compared with controls. As a dramatic effect of exposure to the pesticide, inner plexiform layer (IPL)-like areas in spheroids were not distinguishable anymore. Similarly, photoreceptor rosettes and Müller radial glia were strongly decreased, whereas apoptosis was stimulated. The expression of transcripts for choline-acetyltransferase and muscarinic receptors was affected, revealing an effect of diazinon on the cholinergic system. This further proves the significance of cholinesterases and the cholinergic system for proper nervous system development and shows that further studies of debilitating diazinon actions on development are necessary.

Transdifferentiation of the retinal pigment epithelia to the neural retina by transfer of the Pax6 transcriptional factor

The Pax6 gene plays an important role in eye morphogenesis throughout the animal kingdom. The Pax6 gene and its homologue could form ectopic eyes by targeted expression in Drosophila and Xenopus. Thus, this gene is a master gene for the eye morphogenesis at least in these animals. In the early development of the vertebrate eye, Pax6 is required for the instruction of multipotential progenitor cells of the neural retina (NR). Primitive retinal pigment epithelial (RPE) cells are able to switch their phenotype and differentiate into NR under exogenous intervention, including treatment with fibroblast growth factors (FGFs), and surgical removal of endogenous NR. However, the molecular basis of phenotypic switching is still controversial. Here, we show that Pax6 alone is sufficient to induce transdifferentiation of ectopic NR from RPE cells without addition of FGFs or surgical manipulation. Pax6-mediated transdifferentiation can be induced even at later stages of development. Both in vivo and in vitro studies show that the Pax6 lies downstream of FGF signaling, highlighting the central roles of Pax6 in NR transdifferentiation. Our results provide an evidence of retinogenic potential of nearly mature RPE and a cue for new therapeutic approaches to regenerate functional NR in patients with a visual loss.

Wnt2b controls retinal cell differentiation at the ciliary marginal zone

The ciliary marginal zone of the vertebrate retina contains undifferentiated progenitor cells that continue to proliferate and add new neurons and glia peripherally during the embryonic stages - even after the formation of a functional retina. To understand the molecular mechanism that controls the prolonged progenitor cell proliferation in the ciliary marginal zone, we employed a candidate molecule approach, focusing on Wnt2b (formerly know as Wnt13), which is expressed in the marginal most tip of the retina. Frizzled 4 and 5, seven-pass transmembrane Wnt receptors, were expressed in the peripheral and central part of the retina, respectively. LEF1, a downstream Wnt signaling component, was expressed at high levels in the ciliary marginal zone with expression gradually decreasing towards the central retina. The LEF1-expressing region, which is where Wnt signaling is supposedly activated, expressed a set of molecular markers that are characteristic of the progenitor cells in the ciliary marginal zone. Overexpression of Wnt2b by use of in ovo electroporation in the central retina inhibited neuronal differentiation and induced the progenitor cell markers. Blocking of the Wnt downstream signaling pathway by a dominant-negative LEF1 inhibited proliferation of the cells in the marginal area, which resulted in their premature neuronal differentiation. The progenitor cells in the ciliary marginal zone differentiated into all the neuronal and glial cell types when cultured in vitro, and they proliferated for a longer period than did centrally located progenitor cells that underwent a limited number of cell divisions. In addition, the proliferation of these progenitor cells was promoted in the presence of Wnt2b. These results suggest that Wnt2b functions to maintain undifferentiated progenitor cells in the ciliary marginal zone, and thus serves as a putative stem cell factor in the retina.

Expression of functional N-methyl-D-aspartate receptors during development of chick embryo retina cells: in vitro versus in vivo studies

The N-methyl-D-aspartate (NMDA) ionotropic glutamate receptors were studied in retina cells developing in chick embryos and in retina cells cultured as retinospheroids, at the same stages of development. In the retinospheroids, the activity of the NMDA receptors was followed by monitoring the changes in the intracellular free calcium concentration ([Ca2+](i)), in response to NMDA or to L-glutamate. The expression of the subunits NMDAR1, NMDAR2A/B and NMDAR2C in the retinospheroids and in chick retinas were determined by Western blot analyses. The changes in [Ca2+](i) in response to 400 microM NMDA increased from 5 h in vitro to 3 days in vitro (DIV) and remained constant until 14 DIV, whereas the [Ca2+](i) response to 500 microM L-glutamate increased from 5 h in vitro to 3 DIV and decreased slightly until 14 DIV. In the retinospheroids, the expression of the NMDAR1 and NMDAR2A/B subunits increased from 5 h in vitro until 14 DIV, whereas the NMDAR2C subunit increased from 5 h in vitro until 10 DIV and remained constant until 14 DIV. In the retinas, the expression of NMDAR1 increased from embryonic day 8 (E8) until E15, decreased until E18, and increased again until day 22 (post-hatched 1, PH1). The NMDAR2A/B increased from E8 until E18 and decreased slightly until PH1, whereas the NMDAR2C subunit increased from E8 until E15, remained constant until E18, and increased again until PH1. The results suggest that NMDA receptors are expressed and functionally active at early embryonic stages in the retina and in retinospheroids, before synapse formation (E12). However, the calcium responses to NMDA were relatively constant from 3 DIV until 14 DIV, showing no correlation with the increase in the expression of the studied NMDA receptor subunit during the same period. Also, the patterns of NMDA receptor subunits expressed in chick embryo retina cells cultured in vitro and in retina cells developing in vivo were similar.

3D-biohybrid systems: applications in drug screening

Biotechnology demands powerful methods for the functional characterisation and monitoring of molecular alterations in tissues in response to various stimuli. Currently, cellular biosensors provide information about cell and tissue internal transduction pathways. In this article, recent biosensor systems are briefly described and the use of 3D tissue aggregates as recognition elements is discussed. An example of an innovative approach for drug testing using 3D heart muscle aggregates, as well as tumor models, positioned in capillary systems for electrical potential recording and impedance measurement is described. The effectiveness of drugs and therapies can be tested and monitored in a short time using such biohybrid sensors.

Expression of AMPA/kainate receptors during development of chick embryo retina cells: in vitro versus in vivo studies

The activity and the subunit expression of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)/kainate ionotropic glutamate receptors were studied in retina cells developing in chick embryos and in retina cells cultured as retinospheroids, at the same stages of development. In the retinospheroids, the activity of the AMPA/kainate receptors was monitored by following the changes in the intracellular free calcium concentration ([Ca(2+)](i)), in response to AMPA, kainate or to L-glutamate, and the expression of the receptor subunits GluR1, GluR2/3, GluR4 and GluR6/7 was determined in the retinospheroids and in chick retinas by immunodetection using polyclonal antibodies. The changes in [Ca(2+)](i) in response to 400 microM kainate increased from 5h in vitro to 3 days, and remained constant until day 14, whereas the [Ca(2+)](i) in response to 500 microM L-glutamate or 400 microM AMPA increased from 5h in vitro to 3 days, and thereafter decreased slightly until day 14. The [Ca(2+)](i) responses to kainate are mainly due to AMPA receptor stimulation, since the signals were abolished by LY303070, the AMPA receptor antagonist, and were not affected by MK-801, the NMDA receptor antagonist. In retinospheroids, the levels of expression of GluR1 subunit increased from 5h in vitro until day 7, then decreased until day 14. The levels of expression of GluR2/3 and GluR4 subunits increased from 5h in vitro until day 10, and remained constant until day 14. The levels of kainate receptor subunits GluR6/7 increased from 5h in vitro until day 3, and thereafter decreased slightly until day 14. In the retinas, the expression of GluR1 and GluR6/7 subunits increased from day 8 until day 15, and then decreased until day 22 (post-natal 1). The subunits GluR2/3 and GluR4 increased from day 8 until day 18, and remained constant until day 22. The results suggest that AMPA/kainate receptors are expressed at early embryonic stages, although at low levels and before synapse formation (E12). However, the AMPA receptors are not completely functional at the first stage studied since they do not respond to the agonist AMPA. Also, the patterns of AMPA/kainate receptor subunit expression in retinospheroids of chick embryo retina cells cultured in vitro and in retina cells developing in the embryo (in vivo) were similar, indicating that the AMPA/kainate receptor subunits expression in these primary cultures mimics their expression in the developing chick retina.

From stem cells towards neural layers: a lesson from re-aggregated embryonic retinal cells

Cells from dissociated embryonic avian retinae have the capacity to re-aggregate in rotation culture and form cellular spheres reconstituting a complete arrangement of all retinal layers. This exquisite phenomenon is based upon in vitro proliferation of multipotent precursor stem cells and spatial organization of their differentiating descendants. The addition of soluble factors from cultured retinal pigmented epithelial (RPE) or radial glial cells is essential to revert inside-out spheres (rosetted retinal spheres) into correctly laminated outside-out spheres (stratified spheres). Such complete restoration of a laminated brain tissue by cell re-aggregation has been achieved only for the embryonic avian retina, but not the mammalian retina, nor for other brain parts. This review summarises the history of the re-aggregation approach, presents avian retinal re-aggregate models, and analyses roles of the RPE and Müller cells for successful retinal tissue regeneration. It is predicted that these results will become biomedically relevant, as stem cell biology will soon open ways to produce large amounts of human retinal precursors.

mosaic eyes: a zebrafish gene required in pigmented epithelium for apical localization of retinal cell division and lamination

For proper function of the retina, the correct proportions of retinal cell types must be generated, they must be organized into cell-specific laminae, and appropriate synaptic connections must be made. To understand the genetic regulation of retinal development, we have analyzed mutations in the mosaic eyes gene that disrupt retinal lamination, the localization of retinal cell divisions to the retinal pigmented epithelial surface and retinal pigmented epithelial development. Although retinal organization is severely disrupted in mosaic eyes mutants, surprisingly, retinal cell differentiation occurs. The positions of dividing cells and neurons in the brain appear normal in mosaic eyes mutants, suggesting that wild-type mosaic eyes function is specifically required for normal retinal development. We demonstrate that mosaic eyes function is required within the retinal pigmented epithelium, rather than in dividing retinal cells. This analysis reveals an interaction between the retinal pigmented epithelium and the retina that is required for retinal patterning. We suggest that wild-type mosaic eyes function is required for the retinal pigmented epithelium to signal properly to the retina.

Late proliferation of retinal Müller cell progenitors facilitates preferential targeting with retroviral vectors in vitro

During vertebrate neural retina development, the relationship between mitotic activity in progenitor cells and the acquisition of a mature cell phenotype remains an area of controversy. The Müller glial cell has long been recognized as one of the last cell types of the retina to mature, which occurs under the influence of cell-cell interactions. In this report we examine the acquisition of the Müller cell phenotype in relation to mitotic activity. Using immunohistochemical markers, we demonstrate that a gene product characteristic of mature Müller cells, the 2M6 antigen, is expressed in mitotically active cells, even after all the major retina architectural features have been laid down. Furthermore, we show that retroviral infection, a process that requires mitotically active cells, preferentially targets Müller cell progenitors when late embryonic retina is infected in vitro. The two lines of evidence are consistent with a model for Müller cell differentiation that includes a mitotically active progenitor that has already begun to express specific differentiation gene products.

The key role of butyrylcholinesterase during neurogenesis and neural disorders: an antisense-5'butyrylcholinesterase-DNA study

The wide tissue distribution of butyrylcholinesterase (BChE) in organisms makes specific roles possible, although no clear physiologic function has yet been assigned to this enzyme. In vertebrates, it appears e.g. in serum, hemopoietic cells, liver, lung, heart, at cholinergic synapses, in the central nervous system. in tumors and not at least (besides acetylcholinesterase, AChE) in developing embryonic tissues. Here, a functional role of BChE can be found in regulation of cell proliferation and the onset of differentiation during early neuronal development--independent of its enzymatic activity. For studies concerning this point, we have established a strategy for a specific and efficient inhibition of BChE to investigate how the expected decrease of enzyme and, therefore, the manipulation of cellular cholinesterase-equilibrium influences embryonic neurogenesis--among others to gain information about the significance of noncholinergic, activity-independent and cell growth functions of BChE. The antisense-5'BChE-DNA strategy is based on inhibition of BChE mRNA transcription and protein synthesis. For this, the BChE gene is cloned into a suitable vector system; this is done in antisense-orientation, so that a transfected cell will produce their own antisense mRNA to inhibit gene expression. For such investigations in neurogenesis, the developing retina is a good model and we are able to create organotypic, three-dimensional retinal aggregates in vitro (retinospheroids) using isolated retinal cells of 6-day-old chicken embryos. Using this in vitro retina and "knock out" of BChE gene expression, we could show a key role of BChE during neurogenesis. The results are of great interest because in tumorigenesis and some neuronal disorders, the BChE gene is amplified or abnormally expressed. It has to be discussed how the antisense-5'BChE strategy can play a role in the development of new and efficient therapy forms.

RPE secreted proteins and antibody influence photoreceptor cell survival and maturation

Proteins in media conditioned by retinal pigment epithelial cells (RPE-CM) and an antibody against these proteins (RPE-SP) were tested for their respective effects on rat retinal development in vitro and in vivo. Proteins of RPE-CM were separated in denaturing gels and evaluated by Western blot analysis. Retinal explants from postnatal day 2 (P2) rats were cultured in RPE-CM only or CM diluted with the RPE-SP antibody and, after 7 days, the explants were dissociated into single cells that were immunostained for opsin. RPE-CM or antibody was also injected into the vitreous of postnatal day 7 (P7) Long-Evans rats and analyzed 7 and 21 days later. Electrophoretic analysis of RPE-CM predominantly showed 60-70 kDa proteins; when these proteins were probed with RPE-SP antibody by Western blot, immunoreactive proteins were restricted to this narrow molecular weight range. In P2 retinal explant cultures supplemented with RPE-CM, long ganglion cell-like neurites were detected in 3 days. This activity was nullified in explant cultures grown in RPE-CM titrated with antibody, and these explants appeared to degenerate within 5 days. Over 80% of dissociated retinal cells from explants 7 days after treatment with RPE-CM expressed opsin, compared to only 20% of cells from explants grown in defined medium or serum. Retinas of P14 rats injected intravitreally with RPE-CM at P7 had increased numbers of ectopic photoreceptor cells within the inner nuclear layer when compared to retinas of sham-injected eyes. In contrast, retinas of eyes injected intravitreally with RPE-SP antibody exhibited shorter outer (OS) and inner (IS) segments and thinner outer nuclear (ONL) and outer plexiform (OPL) layers than retinas of sham-injected eyes. In conclusion, proteins in RPE-CM appeared to accelerate and maximize the development of rat photoreceptor cells in vitro, while intravitreal injections of its antibody caused an apparent retardation of outer segment maturation. These results suggest that a protein(s) secreted by RPE plays a key role in normal retinal development, particularly in photoreceptor cell survival and outer segment maturation.

Regulation of cholinesterase gene expression affects neuronal differentiation as revealed by transfection studies on reaggregating embryonic chicken retinal cells

In the embryonic chicken neuroepithelium, butyrylcholinesterase (BChE) as a proliferation marker and then acetylcholinesterase (AChE) as a differentiation marker are expressed in a mutually exclusive manner. These and other data indicate a coregulation of cholinesterase expression, and also possible roles of cholinesterases during neurogenesis. Here, both aspects are investigated by two independent transfection protocols of dissociated retina cells of the 6-day-old chick embryo in reaggregation culture, both protocols leading to efficient overexpression of AChE protein. The effect of the overexpressed AChE protein on the re-establishment of retina-like three-dimensional networks (so-called retinospheroids) was studied. In a first approach, we transfected retinospheroids with a pSVK3 expression vector into which a cDNA construct encoding the entire rabbit AChE gene had been inserted in sense orientation. As detected at the mRNA level, rabbit AChE was heterologously overexpressed in chicken retinospheroids. Remarkably, this was accompanied by a strong increase in endogenous chicken AChE protein, while the total AChE activity was only slightly increased. This increase was due to chicken enzyme, as shown by species-specific inhibition studies using fasciculin. Clearly, total AChE activity is regulated post-translationally. As an alternative method of AChE overexpression, transfection of spheroids was performed with an antisense-5'-BChE vector, which not only resulted in the down-regulation of BChE expression, but also strongly increased chicken AChE transcripts, protein and enzyme activity. Histologically, a higher concentration of AChE protein (as a consequence of either AChE overexpression or BChE suppression) was associated with an advanced degree of tissue differentiation, as detected by immunostaining for the cytoskeletal protein vimentin.

Pigmented epithelium induces complete retinal reconstitution from dispersed embryonic chick retinae in reaggregation culture

Reaggregation of dispersed retinal cells of the chick embryo leads to histotypic retinospheroids in which the laminar organization remains incomplete: photoreceptors form rosettes which are surrounded by constituents of the other retinal layers. Here, for the first time, a complete arrangement of layers is achieved in cellular spheres (stratoids), provided that fully dispersed retinal cells are younger than embryonic day E6, and are reaggregated in the presence of a monolayer of retinal pigmented epithelium (RPE). A remarkable mechanism of stratoid formation from 1 to 15 days in vitro is revealed by the establishment of a radial Müller glia scaffold and of photoreceptors. During the first two days of reaggregation on RPE, rosettes are still observed. At this stage immunostaining with vimentin and F11 antibodies for radial Müller glia reveal a disorganized pattern. Subsequently, radial glia processes organize into long parallel fibre bundles which are arranged like spokes to stabilize the surface and centre of the stratoid. The opsin-specific antibody CERN 901 detects photoreceptors as they gradually build up an outer nuclear layer at the surface. These findings assign to the RPE a decisive role for the genesis and regeneration of a vertebrate retina.

Pigmented epithelium sustains cell proliferation and decreases expression of opsins and acetylcholinesterase in reaggregated chicken retinospheroids

We investigated the effect of the retinal pigmented epithelium on cell proliferation and differentiation in rosetted retinospheroids, which are retina-like spheres reaggregated in the complete absence of retinal pigmented epithelium from dissociated retinal cells of 6-day-old chick embryos in a rotation culture system. In spheroids raised in the absence of retinal pigmented epithelium (controls), acetylcholinesterase was expressed in cells of an inner nuclear-like layer and their neuropil matrices. Moreover, the ratio between rods and cones was found to be approximately normal throughout the spheroid. When spheroids were cultured in the presence of retinal pigmented epithelium monolayers, cell proliferation in spheroids as determined by BrdU labelling was significantly increased and extended for 1 week, while acetylcholinesterase protein levels and specific activities in homogenates were decreased to approximately 30%. At the same time, opsin immunoreactivity was completely suppressed within the spheroid and appeared slowly in cells around its periphery; i.e. the proportion of rhodopsin-positive cells decreased from 14 to 3%. This study reveals that the retinal pigmented epithelium in vitro sustains cell proliferation but inhibits the differentiation of acetylcholinesterase-positive cells and of photoreceptors.

On-line detection of glutamate release from cultured chick retinospheroids

A continuous fluorometric assay was adapted to measure the release of endogenous glutamate from cultured chick retinospheroids. The results obtained with this technique are compared with the release of [3H]D-aspartate from monolayer cultures of chick retina cells. It is shown that although excitatory amino acids may be released in a Ca(2+)-dependent manner, most of the neurotransmitter release from cultured retina cells occurs by reversal of the glutamate transporter. The presence of extracellular Ca2+ may actually inhibit glutamate release by the cells present in the retinospheroids, or the [3H]D-aspartate release by cells in monolayers, when veratridine is the depolarizing agent.

Differential effects of bFGF on development of the rat retina

A variety of growth factors can influence the expression of differentiated properties by cell types of the developing retina. One unresolved question has been whether these factors can direct the differentiation pathway of uncommitted precursors or whether they act to help the expression of properties by already committed cells. To address this question we have studied the effects of basic fibroblast growth factor (bFGF) on the differentiation of ganglion cells and rod photoreceptors in explant cultures of embryonic rat retinas. Incubation of retinas in the presence of bFGF accelerated the appearance of differentiated ganglion cells and incubation in the presence of anti-bFGF antibodies delayed the appearance. bFGF had no effect on the appearance of differentiated rod photoreceptors as judged by expression of opsin, although all-trans-retinoic acid did increase the number of cells expressing opsin. bFGF inhibited the formation of rod photoreceptor rosettes suggesting that it does influence some properties of rods or the adjacent Müller glial cells. The results suggest that bFGF can alter the timing of differentiation of retinal ganglion cells but not direct their production from retinal precursors.

Glutamate release evoked by glutamate receptor agonists in cultured chick retina cells: modulation by arachidonic acid

We studied the effect of ionotropic glutamate receptor agonists on the release of endogenous glutamate or of [3H]D-aspartate from reaggregate cultures (retinospheroids) or from monolayer cultures of chick retinal cells, respectively. Kainate increased the fluorescence ratio of the Na+ indicator SBFI and stimulated a dose-dependent release of glutamate in low (0.1 mM) Ca2+ medium, as measured using a fluorometric assay. Under the same experimental conditions, the release evoked by N-methyl-D-aspartate (NMDA; 400 microM) was about half of that evoked by the same kainate concentration; alpha-amino-3-hydroxy-5-methyl-4-isoxasolepropionic acid (AMPA; 400 microM) did not trigger a significant response. In the presence of 1 mM CaCl2, all of the agonists increased the [Ca2+]i, as determined with the fluorescence dye Indo-1, but the glutamate release evoked by NMDA and kainate was significantly lower than that measured in 0.1 mM CaCl2 medium. Inhibition by Ca2+ of the kainate-stimulated release of glutamate was partially reversed by the phospholipase A2 inhibitor oleiloxyethyl phosphorylcholine (OPC), suggesting that the effect was mediated by the release of arachidonic acid, which inhibits the glutamate carrier. Accordingly, kainate, NMDA, and AMPA stimulated a Ca(2+)-dependent release of [3H]arachidonic acid, and the direct addition of the exogenous fatty acid to the medium decreased the release of glutamate evoked by kainate in low (0.1 mM) CaCl2 medium. In monolayer cultures, we showed that NMDA, kainate, and AMPA also stimulated the release of [3H]D-aspartate, but in this case release in the presence of 1 mM CaCl2 was significantly higher than that evoked in media with no added Ca2+. The ranking order of efficacy for stimulation of Ca(2+)-dependent release of [3H]D-aspartate was NMDA > > kainate > AMPA.

The retinal pigmented epithelium is required for development and maintenance of the mouse neural retina

BACKGROUND

During development of the vertebrate eye, there is a series of reciprocal cellular interactions that determine the fate of the eye components. Although evidence from organ culture suggests that the retinal pigmented epithelium (RPE) organizes the laminar structure of the differentiated neural retina, no role has been identified for the RPE in early eye development, nor has the later function of RPE been demonstrated in vivo.

RESULTS

To investigate the role of RPE cells in eye development, we generated transgenic mice that carry the attenuated diphtheria toxin-A gene; this transgene was driven by the promoter of the gene encoding the tyrosinase-related protein-1, which is specifically expressed in pigment cells. Depending on the expression level of the transgene, the retinal epithelium was ablated before or after its differentiation into a pigmented cell layer. We show that an early ablation (embryonic day E10-11) resulted in disorganization of the retinal layer, immediate arrest of eye growth and subsequent eye resorption. A later ablation (E11.5-12.5) allowed the eye to be maintained during embryogenesis, but the laminar structure of the retina became disrupted by the end of gestation, the vitreous failed to accumulate the adults were anophthalmic or severely microphthalmic. In some microphthalmic eyes, a number of RPE cells escaped ablation and formed patches of pigmented cells; the laminar structure of the retina was maintained immediately adjacent to such pigmented areas but disrupted elsewhere. In both cases--early or late ablation of the RPE--the retina appears to be the primary affected tissue.

CONCLUSIONS

We conclude that presence of the RPE is required for the normal development of the eye in vivo. Its presence early in development is necessary for the correct morphogenesis of the neural retina. After the neural retina has started to differentiate, the RPE is still necessary, either directly or indirectly, to maintain the organization of the retinal lamina.

Co-transplantation of embryonic retina and retinal pigment epithelial cells to rabbit retina

The retinal pigment epithelium (RPE) is important for normal development of the neural retina. We sought to investigate whether cografting RPE cells affected the differentiation and survival of retinal grafts. Pigmented embryonic day 16 (E16) rabbit retina was dissected either with or without attached RPE and injected into a lesion site in retinas of young adult rabbit hosts. Each host obtained a pure retina graft in one eye and a retina/RPE cograft in the other. Animals were sacrificed after 4, 8 and 12 weeks. After 4 weeks, grafts (1-2 mm in diameter) were seen in both experimental groups at the lesion site or in the subretinal space. However, 8 and 12 weeks after transplantation, the graft survival rate decreased. The grafts developed cell layers in folded sheets and many rosettes (a rosette consists of photoreceptors and cells of other retinal layers around a central lumen defined by an outer limiting membrane). Cografts of retina with RPE had areas of more distinct cell lamination than transplants of pure retina. Grafted RPE cells were organized in clusters of cells surrounded by extracellular matrix and often associated with blood vessels. If the extracellular matrix of RPE cell clusters was outside the rosettes close to inner retinal layers in the graft, transplant Müller cell endfeet developed an inner limiting membrane. Müller cell endfeet could also be observed in subretinal transplants attached to the denuded Bruch's membrane of the host. In 12-week grafts, when RPE cell clusters were inside rosettes, the surrounded photoreceptors survived better. No RPE effect could be seen if single RPE cells were dispersed among retinal donor cells.(ABSTRACT TRUNCATED AT 250 WORDS)